tests: drivers: build_all: Rework modem test to be more generic

Add support to build all current modem drivers (WNCM14A2A, GSM PPP, and
ublox-sara).  We need a unique config for ublox-sara to be built since
the networking params conflict with other modem drivers.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>