This is the radio found in NXP Kinetis KW41Z, KW21Z. Only 802.15.4 mode
is implemented (KW41Z also supports BLE on the same transceiver).
The driver uses vendor supplied initialization code for the low level
XCVR hardware, these files were imported from KSDK 2.2.0 (framework_5.3.5)
cpu/$(CPU)/Makefile.features and cpu/$(CPU)/Makefile.dep are
automatically included
Part of moving CPU/CPU_MODEL definition to Makefile.features to have it
available before Makefile.include.