Mark3 RTOS Source

-Adding notify tests to simulator test script.

Getting rid of the usage of K_ types, in favor of using those found in <stdint.h>... it's standard for a reason.

Adding Software UART implementation (Transmit only -- good for debugging).
Tested to 115200 with an FTDI cable.

-- Adding some helpful APIs to the Mailbox class to get the free envelope count, etc.
-- Minor refactoring to the heap code (arena + fixed_block heaps).

updating arena allocator documentation

-Fixing arena allocator compilation for AVR. Still needs to be tested.

-Fix up some formatting in kernel makefile, and adding mailbox.h to the mark3.h header
-Also removing timer_precision unit test from regular tests, since there's something funky with it.

Fix mailbox unit test after adding send-block tests.

Adding mailbox unit-test.

-Adding mailbox blocking-send calls
-Reorganized the timer-tick MAX_TIMER_TICKS define
-Fixed some bugs in the mailbox implementation