lorawan: enable run-time config of region/freq

This commit adds support for compiling in support for several different
regions/frequencies and dynamically choosing which to use in run-time.
This commit introduces no API breakages - if a prj.conf contains only a
single region Kconfig, the new function lorawan_set_region() does not
need to be called.

Signed-off-by: Benjamin Lindqvist <benjamin@eub.se>