Instead of OOCD_SERIAL triggering an attempt to use a specific ftdi based
adapter, introduce common target/interface pairs, OOCD_INTERFACE and
OOCD_TARGET, and an override via OOCD_FILE allowing full control.
Further, attempt to connect to a running openocd instance to flash if possible.
This _will not_ work well if you have multiple openocd instances open at once,
but that's a rare use case.
Examples:
OOCD_INTERFACE=stlink-v2
OOCD_TARGET=stm32f0x
--> openocd ... -f interface/stlink-v2.cfg -f target/stm32f0x.cfg)
OOCD_FILE=board/ek-lm4f120xl.cfg
--> openocd ... -f $(OOCD_FILE) (ie, the variable is passed untouched)
As part of implementing this, correct variables that were always wrong, and
update the openocd flashing commands to use current commands.
