fstyp: detect Raspberry Pi Pico boot filesystem as FAT

fstyp looks for a 0x55 0xAA signature at offset 510, but this is not
required by specifications and is not proivded by the Raspberry Pi Nano
bootloader.

We should really remove the signature check and implement a more
comprehensive BPB validation instead, but it will require more
investigation and testing. For now just add a special case for the
Raspberry Pi Nano bootloader, to avoid introducing regressions or new
false positives.

PR: 262896
Reviewed by: delphij
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34699