Support for A/B FSG partition

[jackmthws]

Some/all Motorola A/B devices have A/B fsg partitions, fsg_a and fsg_b. RMTFS does not have code to handle this, so when the modem requests the fsg partition, RMTFS fails to provide it and the modem crashes.

A quickly hacked version, hardcoding RMTFS to use one of the two, works on both motorola-payton (moto x4) and motorola-beckham (moto z3 play)

[Kiciuk]

Bumping, it would be cool to get it done(motorola-deen)

[quic-kdybcio]

Please see the commit that GH conveniently referenced above, I only compile-tested it though.

[jackmthws]

Please see the commit that GH conveniently referenced above, I only compile-tested it though.

Tested given commit, it works :) Thank you so much