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.

config/zinit/init/firmware.sh

@@ -1,5 +1,5 @@
 #!/bin/sh
-# rfs mount firmware flist over /usr/lib/firmware (plain S3 route inside the .fl)
+# rfs mount firmware flist over /lib/firmware (plain S3 route inside the .fl)
 # Looks for firmware-latest.fl in known locations; can be overridden via FIRMWARE_FLIST env
 
 set -eu
@@ -7,7 +7,7 @@ set -eu
 log() { echo "[rfs-firmware] $*"; }
 
 RFS_BIN="${RFS_BIN:-rfs}"
-TARGET="/usr/lib/firmware"
+TARGET="/lib/firmware"
 BASE_URL="${FLISTS_BASE_URL:-https://zos.grid.tf/store/flists}"
 
 # Allow override via env