Add some commands to the GDB stub

I've got all IAR built binaries and have been hoping to try to figure out a way to convert the C-Spy files (UBROF file format with .d90 extension) to something that I can work with in GDB or getting IAR Embedded Workbench to talk to a GDB stub but alas no luck. But I did learn a little bit of GDB stub/serial protocol bits and implemented some commands I noticed that my copy of avr-gdb requested but we reported as unsupported.