Temp disable BL tests during Thunk2Real

I believe that there is an issue in MRI which needs to be corrected so
that running these tests doesn't leave the debug monitor in a weird
state after executing the tests which set PC to an invalid address.
Add serial logging to commSerial

Currently is is disabled but setting SERIAL_LOG to non-zero value will
cause it to be turned on and serial I/O will be logged to serial.log.
The output is similar to that used by GDB remote target logging.
Disable 8-byte stack alignment in realarm

There appears to be a problem with MRI, context changing of SP register
and 8-byte alignment during debug monitor exception.  I have disabled
this feature in the Cortex-M3 for now.  Can switch it back later after
investigating the MRI issue.
Add thunk2real tests

Building and running thunk2real causes a good portion of the simulator
unit tests to be marshalled over to an mbed to run on real ARM
hardware.
Ignore serial.log output file