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 provided by the Raspberry Pi Pico
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 Pico 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

(cherry picked from commit 868c1b8431f297ade8deba5baf903f73cf5e11c6)
(cherry picked from commit 27c2f016b86744aa5d4c6031b4ef2fc16bbf6546)
(cherry picked from commit e06ce938ddc0222c6ed10a70108f71685923bd45)

Approved by: re (gjb)