mkfs-util: drive the vfat mtools recursion ourselves for reproducibility

mkfs-util: drive the vfat mtools recursion ourselves for reproducibility

do_mcopy() invoked 'mcopy -s' with the top-level sources sorted by name,
but then relied on mtools' built-in recursion to walk into subdirectories.
mtools uses platform readdir(), so the order in which it inserts entries
into the freshly-formatted FAT image inherits whatever order the source
filesystem returns. With an ext4 staging directory the directory hash
changes the readdir order between hosts (and between runs after edits),
and because FAT stores directory entries in insertion order the resulting
ESP differs byte-for-byte across otherwise identical builds.

Stop using 'mcopy -s'. Walk the source ourselves with
readdir_all_at(RECURSE_DIR_SORT) at every level and emit per-item mtools
commands interleaved per parent: each subdirectory is created with 'mmd'
before the recursion descends into it, and consecutive sibling files are
batched into a single 'mcopy' invocation so the fork cost stays
proportional to the number of directory transitions rather than to the
total number of files.

Combined with the existing 'mkfs.fat --invariant' and SOURCE_DATE_EPOCH
plumbing, this makes vfat populated via repart byte-stable regardless of
the staging filesystem's readdir() behaviour.

Co-developed-by: Claude Opus 4.7 <noreply@anthropic.com>
Signed-off-by: Paul Meyer <katexochen0@gmail.com>