rfs(firmware): pack full Alpine linux-firmware set from container and overmount /lib/firmware

Change pack source: install all linux-firmware* packages in container and pack from /lib/firmware via [bash.rfs_common_install_all_alpine_firmware_packages()](scripts/rfs/common.sh:290) used by [bash.pack-firmware.sh](scripts/rfs/pack-firmware.sh:21). At runtime, overmount firmware flist on /lib/firmware by updating [sh.firmware.sh](config/zinit/init/firmware.sh:10). Update docs to reflect /lib/firmware mount and new pack strategy.

docs/rfs-flists.md

@@ -15,9 +15,10 @@ Scope of this change
 Inputs
 - Built kernel modules present in the dev-container (from kernel build stages):
   - Preferred: /lib/modules/KERNEL_FULL_VERSION
-- Firmware tree:
-  - Preferred: $PROJECT_ROOT/firmware (prepopulated tree from dev-container: “$root/firmware”)
-  - Fallback: initramfs/lib/firmware created by apk install of firmware packages
+- Firmware source for RFS pack:
+-   Install all Alpine linux-firmware* packages into the build container and use /lib/firmware as the source (full set).
+- Initramfs fallback (build-time):
+-   Selective firmware packages installed by [bash.alpine_install_firmware()](scripts/lib/alpine.sh:392) into initramfs/lib/firmware (kept inside the initramfs).
 - Kernel version derivation (never use uname -r in container):
   - Combine KERNEL_VERSION from [config/build.conf](config/build.conf) and LOCALVERSION from [config/kernel.config](config/kernel.config).
   - This matches [kernel_get_full_version()](scripts/lib/kernel.sh:14).
@@ -71,7 +72,7 @@ Scripts to add (standalone)
 
 Runtime (deferred to a follow-up)
 - New zinit units to mount and coldplug:
-  - Mount firmware flist read-only at /usr/lib/firmware
+  -   Mount firmware flist read-only at /lib/firmware
   - Mount modules flist at /lib/modules/KERNEL_FULL_VERSION
   - Run depmod -a KERNEL_FULL_VERSION
   - udevadm control --reload; udevadm trigger --action=add; udevadm settle