stmhal: Add pyb.wfi() function (calls __WFI).
stmhal: Improved usart module a bit.
stmhal: Work around crazy bug in USB CDC.

Packets of 64 bytes length are not send to the host until the following
packet is sent.  Fixed by never sending packets of 64 bytes length.
tools: Improve timout/reading of pyboard.py.
Merge branch 'master' of github.com:micropython/micropython
Merge branch 'master' of github.com:micropython/micropython
Make USE_COMPUTED_GOTO a config option in mpconfig.h.

Disabled by default.  Enabled in unix port.