Tweak event and sleep ARM intrinsics to indicate they change memory.
Add isb, dsb, dmb ARM intrinsics.
Add Linux kfifo implementation, rejiggered to work on embedded platforms without user/kernel memory distinction, memory alloc/free, DMA support, etc.
Also bring in required bits from Linux #include headers.
Remove min() definition, replace with Linux min() in linux_stuff.h.
Remove now-unused #includes.
Simple IPC mechanism for M0 UI requesting RF/DSP operations from M4 core.
Adapt UI and DSP code to use IPC for RF/DSP operations.
Rework set_frequency() to return success from set_freq(), not do its own frequency range test..