Just low-level read/write support.  No filesystem yet.

stmhal - Add usart support

Remove unnecessary includes. Add includes that improve portability.

Remove unnecessary includes. Add includes that improve portability.

New USB HAL is quite a bit improved over previous one.  Now has better
callbacks and flow control.

REPL over USB CDC now works as before, except for soft-reset (since USB
driver uses malloc...).

Either not needed anymore, or defined in the board/_hal_conf.h file.

stmhal - add led support. Add netduino_plus_2 support

Tweaked a couple of the USB files to compile if neither dev nor host was defined.
Tested on netduiono plus 2 and stm32f4discovery boards

Conflicts:
	stmhal/main.c

Enumerates CDC device over USB, but doesn't transmit/receive yet.

stmhal - More systick cleanup. Fix HAL_Delay

Implement str.count and add tests for it.

Cleanup systick for stmhal

REPL working on UART6 with STMHAL

See pull request #343.

Also modify mp_get_index to accept:
1. Indices that are or evaluate to a boolean.
2. Slice indices.
Add tests for these two cases.