linux-ampere-fourier: bootstrap PKGBUILD (CoolPi GenBook RK3588)

Sibling to kernel-agent PR #8 which lands the scope-tagged board
patches + fleet/ampere.yaml manifest. This PR carries the
makepkg-side recipe.

## Baseline

marfrit/linux-rk3588-marfrit @ f8f3ad9 (mainline v7.0-rc3 + 18 commits):
  10 by Markus Fritsche  (board DTS + Kconfig + suspend/wakeup + bt)
   4 by Shawn Lin        (phy: rockchip-snps-pcie3 stability)
   2 by Cristian Ciocaltea (clk + dw-dp bridge — Collabora)
   1 by Sebastian Reichel (Rock 5 ITX hdmirx — unused by ampere)
   1 by Pedro Alves      (CLK_SET_RATE_PARENT VOP2 fix)

The 6 board-relevant patches (pwm15 pinctrl + RK806 power-off +
genbook pwm-fan/speaker/USB-C/lid) are scope-tagged in
kernel-agent. The 12 others are 'cherry-picks-in-baseline'
currently — splitting them into scope-tagged patches is a
follow-up.

## Source distribution

The kernel tree isn't pushable to Gitea (boltzmann's working clone
is shallow). Tarball (260MB) doesn't belong in marfrit-packages
either. Pragmatic shortcut: ship prebuild.sh that produces the
tarball locally from $LINUX_RK3588_MARFRIT_TREE (default
~/src/linux-rockchip) just before makepkg runs. PKGBUILD source
array references the tarball by name; makepkg picks it up from
the PKGBUILD dir.

Build flow:
  cd arch/linux-ampere-fourier
  ./prebuild.sh           # produces 260MB tarball; idempotent w/ sha check
  makepkg -s --noconfirm

Future iteration: host the tarball on Gitea release assets or
packages.reauktion.de/sources/ and switch source to URL.

## What the PR ships

  PKGBUILD                          — the recipe (linux-rk3588-marfrit @ f8f3ad9)
  config                            — snapshot of ampere's /proc/config.gz
  linux-ampere-fourier.preset       — mkinitcpio preset, /boot/firmware/ paths
  extlinux-add.hook                 — alpm hook, fires on /boot/firmware/Image* changes
  extlinux-add.sh                   — idempotent managed-label injector for
                                     /boot/firmware/extlinux/extlinux.conf
                                     (vfat on mmcblk0p1, ampere-specific path)
  prebuild.sh                       — source-staging helper (see above)
  README.md                         — build + install runbook

## ampere-specific boot path differences vs fresnel

ampere boots from /boot/firmware/ (vfat partition), not /boot/ (root
partition) like fresnel. The PKGBUILD installs Image + initramfs + DTB
under /boot/firmware/ directly. The extlinux-add hook fires on
/boot/firmware/<X> path changes and edits
/boot/firmware/extlinux/extlinux.conf. The mkinitcpio preset writes
initramfs to /boot/firmware/.

Only one PRESET ('default') — no fallback image — because
/boot/firmware is ~1G total and two ~20MB initramfs files plus the
DTB and a couple of kernel-image backups would squeeze it tight.

## Out of scope this PR

- Ask 2 (VP9 enablement) and Ask 3 (AV1 dec) from kernel-agent issue #6
- Build + publish + install (sequenced after this PR + kernel-agent PR #8 merge)
- Splitting the 12 non-board commits into scope-tagged kernel-agent patches

arch/linux-ampere-fourier/extlinux-add.sh

@@ -0,0 +1,62 @@
+#!/bin/sh
+# Add / update / remove the linux-ampere-fourier entry in
+# /boot/firmware/extlinux/extlinux.conf. Idempotent. Coexists with
+# the hand-managed labels in that file; never edits them. Default
+# label is NOT touched — user picks at u-boot menu.
+#
+# ampere boots from a vfat partition (mmcblk0p1) mounted at
+# /boot/firmware/, distinct from fresnel's /boot/ on root.
+
+set -eu
+
+CONF="/boot/firmware/extlinux/extlinux.conf"
+TAG_BEGIN="# >>> linux-ampere-fourier (managed) >>>"
+TAG_END="# <<< linux-ampere-fourier (managed) <<<"
+
+# Copy APPEND from the user's `arch_mainline` label so the managed
+# entry inherits the same root= and console= settings the host's
+# bootloader already trusts. Falls back to a CoolPi GenBook default
+# if no arch_mainline label exists (first-time install on a fresh
+# bootloader config).
+EXISTING_APPEND=$(awk '
+  /^[[:space:]]*label[[:space:]]+arch_mainline[[:space:]]*$/ { found=1; next }
+  found && /^[[:space:]]*append[[:space:]]/ {
+    sub(/^[[:space:]]*append[[:space:]]+/, "")
+    print
+    exit
+  }
+  /^[[:space:]]*label[[:space:]]/ { found=0 }
+' "$CONF" 2>/dev/null || true)
+
+APPEND="${EXISTING_APPEND:-root=LABEL=arch rw rootwait rootfstype=btrfs rootflags=subvol=@,ssd,discard=async console=ttyS2,1500000 console=tty1 consoleblank=0 loglevel=7 cma=256M coherent_pool=2M}"
+
+ENTRY=$(cat <<EOF
+${TAG_BEGIN}
+label linux-ampere-fourier
+	menu label linux-ampere-fourier (managed)
+	kernel /Image-ampere-fourier
+	initrd /initramfs-ampere-fourier.img
+	fdt    /rk3588-coolpi-cm5-genbook.dtb-ampere-fourier
+	append ${APPEND}
+${TAG_END}
+EOF
+)
+
+# Strip any prior managed block, then append fresh
+TMP=$(mktemp)
+awk -v b="$TAG_BEGIN" -v e="$TAG_END" '
+  $0==b{skip=1; next}
+  $0==e{skip=0; next}
+  !skip{print}
+' "$CONF" > "$TMP"
+
+# Post-Remove: kernel files absent → don't re-add the entry
+if [ -f "/boot/firmware/Image-ampere-fourier" ] \
+   && [ -f "/boot/firmware/rk3588-coolpi-cm5-genbook.dtb-ampere-fourier" ]; then
+  printf '%s\n' "$ENTRY" >> "$TMP"
+  echo "linux-ampere-fourier: extlinux entry updated"
+else
+  echo "linux-ampere-fourier: kernel files absent, entry removed"
+fi
+
+mv "$TMP" "$CONF"