nanobsd: embedded: Calculate FAT type and scheme

[jlduran]

Summary

Determine the FAT (MS-DOS) type (FAT32, FAT16, or FAT12) and partition
scheme (fat32lba or fat16b) based on partition size.

Accept any (NetBSD) strsuftoll(3)-compatible string, as the value will
be fed to makefs(8) "-s" in a future commit.

NANO_SLICE_FAT_SIZEs defined in sectors (ending with an "s") will not be
supported. As a workaround, the letter "b" (blocks) may be used.

[jlduran]

• Implement a strsuftoll(3)-compatible function, as the value of NANO_SLICE_FAT_SIZE should be compatible with makefs(8) -s.