fs: fcb: add option to disable CRC for fcb entries

Add option to disable CRC for fcb entries. This improves the write
throughput significantly at the cost of not detecting corrupted data
in flash. This is beneficial for aplications that needs the extra
write throughput, where error detection is done elsewhere.

Allow the FCB entries in flash to have a valid CRC when CRC is
disabled in the FCB. This allows existing solutions to disable
CRC checking, while keeping the CRC areas intact. Note that this
is a one-way option.

Fixes #53707

Signed-off-by: Eivind Jølsgard <eivind.jolsgard@nordicsemi.no>