Added RTS/CTS flow control

Currently implemented only for LPC1768. On this platform, when hardware
flow control is not directly supported, it will be emulated.
Also added "not_implemented.c" as a placeholder for various HAL functions
that might not be implemented on all platforms (in this particular case,
serial_set_flow_control). These are weak implementations that default to a
"not implemented" error message.
Fixed RTS/CTS bit-banging on LPC1768
Added flow control test

Since this requires a separate serial port connection, added this as a
new attribute of the MUTs.
Hardware flow control implementation for LPC81X
Added DEVICE_SERIAL_FC as an indicator for the flow control implementation
LPC1768 flow control fixes

- Disable TX buffer, this isn't compatible with the software CTS implementation
- Properly set hardware RTS/CTS pins when possible
- Modified test to use hardware CTS and software RTS
Merge branch 'flow_control'

Conflicts:
	workspace_tools/tests.py
Always use the local version of workspace_tools

This ensures compatibility with the mbed-tools package.
Added preliminary printf() support to RawSerial
Changed flow control test pin assignments