Make mksquashfs generate a fs_config-friendly root entry

This change adds a flag so that the mksquashfs wrapper scripts generate
an entry that is fs_config-friendly for the root inode. Namely, the root
entry is expected to have an empty filename.

Bug: 72745016
Test: m -j100 dist
Change-Id: Iebdf79c5af0b9d999b7e5f5fe240abfe52cbadda