Add support to MSDOS FS in PPC loader

Although PPC OFW loader already had a LOADER_MSDOS_SUPPORT option, a few lines
were missing in conf.c, in order to support FAT filesystems.

This is useful when running FreeBSD under QEMU, to be able to easily change the
kernel and modules when running on hosts without UFS read/write support.

Reviewed by: jhibbits
Sponsored by: Eldorado Research Institute (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D24328