This provides a HAL layer for Modem bearing devices.
Provides a standard interface to upper layer drivers.
Platform providers will be implementing this API under their
specific targets.
As a reference, two implementations are provided under TARGET_C027 (UBLOX)
and TARGET_MTS_DRAGONFLY_F411RE (MultiTech).
targets.json now contains a tag "MODEM" which tells that this target
has a modem and the modem_api is protected by a flag DEVICE_MODEM
(following the DEVICE_SERIAL fashion ).
Hasnain Virk