make flash: overhaul openocd configuration

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.

examples/tiva/lm4f/stellaris-ek-lm4f120xl/miniblink/Makefile

@@ -18,7 +18,7 @@
 ##
 
 BINARY = miniblink
-
+OOCD_FILE = board/ek-lm4f120xl.cfg
 LDSCRIPT = ../ek-lm4f120xl.ld
 
 include ../../Makefile.include

examples/tiva/lm4f/stellaris-ek-lm4f120xl/uart_echo_interrupt/Makefile

@@ -18,7 +18,7 @@
 ##
 
 BINARY = uart_echo_interrupt
-
+OOCD_FILE = board/ek-lm4f120xl.cfg
 LDSCRIPT = ../ek-lm4f120xl.ld
 
 include ../../Makefile.include

examples/tiva/lm4f/stellaris-ek-lm4f120xl/uart_echo_simple/Makefile

@@ -18,7 +18,7 @@
 ##
 
 BINARY = uart_echo_simple
-
+OOCD_FILE = board/ek-lm4f120xl.cfg
 LDSCRIPT = ../ek-lm4f120xl.ld
 
 include ../../Makefile.include

examples/tiva/lm4f/stellaris-ek-lm4f120xl/usb_bulk_dev/Makefile

@@ -18,7 +18,7 @@
 ##
 
 BINARY = usb_bulk_dev
-
+OOCD_FILE = board/ek-lm4f120xl.cfg
 LDSCRIPT = ../ek-lm4f120xl.ld
 
 include ../../Makefile.include

examples/tiva/lm4f/stellaris-ek-lm4f120xl/usb_to_serial_cdcacm/Makefile

@@ -18,7 +18,7 @@
 ##
 
 BINARY = usb_to_serial_cdcacm
-
+OOCD_FILE = board/ek-lm4f120xl.cfg
 LDSCRIPT = ../ek-lm4f120xl.ld