Changed SerialConsole definition for MRI/Debug UART so it supports all 4...

This is an update of the debug serial pin-assignment code. This way the user can select all 4 locations (listed in the "mri/mri.h" file) in src/makefile by changing the macro:
''' 
"MRI_UART=MRI_UART_0"
'''

This way the user has more freedom on configuring smoothie. I needed this because I used a custom LPCXPresso->Pololu drivers board, which unluckily uses both UART0 and UART1 pins so I mapped it to UART2.