.github: drop rootfs.squashfs from release tarballs

Release tarballs waste ~131MB per architecture by including both
rootfs.itb and rootfs.squashfs, even though rootfs.itb already
contains the squashfs (itb = 4096-byte header + squashfs).

This change removes rootfs.squashfs from releases and provides two
solutions for users who need it:

1. utils/extract-squashfs.sh - Full-featured utility for extracting
   squashfs from release tarballs or .itb files. Includes validation
   and helpful error messages.

2. board/common/qemu/qemu.sh - Auto-extraction support. When running
   in initrd mode, automatically extracts rootfs.squashfs from
   rootfs.itb if missing. Zero user impact - ./qemu.sh just works!

Space savings: ~262MB per release (2 architectures × 131MB)

Fixes #858

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

.github/workflows/build-release.yml

@@ -100,6 +100,10 @@ jobs:
           mv cpe.json         images/sbom/
           mv legal-info/*.csv images/sbom/
 
+          # Remove rootfs.squashfs from release (can be extracted from rootfs.itb)
+          # Saves ~131MB per architecture. See issue #858
+          rm -f images/rootfs.squashfs
+
           mv images ${{ steps.vars.outputs.dir }}
           ln -s ${{ steps.vars.outputs.dir }} images
           tar cfz ${{ steps.vars.outputs.tgz }} ${{ steps.vars.outputs.dir }}