scripts/gen_image_generic.sh: fix order of files in EFI bootfs

scripts/gen_image_generic.sh: fix order of files in EFI bootfs

mtools recursive copy (mcopy -s ...) is using READDIR(3) to iterate
over the directory entries, hence they end up in the FAT filesystem in
traversal order which breaks reproducibility (rather than being added
to the FAT filesystem in a reproducible order). Implement recursive
copy in gen_image_generic.sh in Shell code instead, as in that way we
can force files to be copied in reproducible order.

Fixes: aece8f5ae8 ("scripts/gen_image_generic.sh: generate reproducible EFI filesystem")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>