sys/suit: save seq_number when parsing manifest

[fjmolinas]

Contribution description

At the end of an update the manifest seq_no should set "somewhere". For firmware update, this is done implicitly since the seq_no is contained in the firmware update. But for other storage units, it needs to be obtained from the manifest.

Therefore when parsing the manifest save the sequence number, and for ram storage set it during install.

Testing procedure

In master, the sequence_no is never checked when using RAM storage, with this PR it is (although only ephemerally). Follow the README.native.md

Or alternatively, leverage #16771.