nanobsd: embedded: Remove mtools dependency

Rely on makefs -t msdos to create the MS-DOS partition and remove the
dependency on filesystems/mtools.

Note that NANO_SLICE_FAT_SIZEs defined in sectors (ending with an "s")
are no longer supported, as a workaround the letter "b" (blocks) may be
used instead.

Reviewed by: imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D54976