Replace direct accesses to sched_active_thread and sched_active_pid with
the helper functions thread_getpid() and thread_get_active(). This serves
two purposes:
1. It makes accidental writes to those variable from outside core less likely.
2. Casting off the volatile qualifier is now well contained to those two
functions
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)
