gdb: move baud_rate and serial_parity declarations to serial.h

They are currently in target.h, it would make more sense to have them in
serial.h, since they are defined in serial.c.

gdb/ChangeLog:

	* target.h (baud_rate, serial_parity): Move declarations...
	* serial.h: ... here.
	* main.c: Include serial.h.
	* serial.c (baud_rate, serial_parity): Update doc.

Change-Id: Idc983c154c80ccc29b07ce68df3483cefe03fb71