mkfs-util: Add support to populate vfat without mounting using mcopy

mkfs-util: Add support to populate vfat without mounting using mcopy

mkfs.vfat doesn't support specifying a root directory to bootstrap
the filesystem from (see https://github.com/dosfstools/dosfstools/issues/183).
Instead, we can use the mcopy tool from the mtools package to copy
files into the vfat filesystem after creating it without needing to
mount the vfat filesystem.