modem/*: modernize and clean up APKBUILDs

Currently contains !502 (closed)

Changes in rmtfs should be tested on more devices, works fine on the Fairphone 2.

The changes in command line parameters of rmtfs mean: use the modem* partitions directly (-P), in read-only mode (-r)...and it will boot the modem automagically when ready (-s)

I've removed the udev rules in the rmtfs package because it doesn't look like they are needed with those command line flags.