modem: get IMSI from ofono

There's no need to specify the IMSI manually in resource config
and it's also prone to errors. Let's take it from ofono.
Add a 'sim' feature to allow modem to auto-discover it,
otherwise if not supported leave that feature out of the config for that
modem and an imsi can still be manually providen.

Change-Id: I20f9e8d97775293925205e4ea576d814214bf1a8
3 files changed