Add a working OpenOCD config for JTAG debugging

firmware/openocd.cfg

+#
+# OpenOCD configuration for JTAG debugging on the Kinetis K20.
+#
+# Use the OpenOCD patch from:
+#   https://forum.sparkfun.com/viewtopic.php?f=18&t=36393
+#
+#   # checkout code from OpenOCD master branch
+#   git clone git://git.code.sf.net/p/openocd/code openocd
+#
+#   # apply patch, this info is given on the gerrit page for the patch.
+#   git fetch http://openocd.zylin.com/openocd refs/changes/08/1508/1 && git checkout FETCH_HEAD
+#
+#   ./bootstrap
+#   ./configure --enable-ftdi
+#   make
+#
+
+source [find interface/ftdi/olimex-arm-usb-ocd-h.cfg]
+
+# Freescale Kinetis K20 device. Should be compatible with K40,
+# just set a custom name.
+set CHIPNAME k20
+source [find target/k40.cfg]
+
+adapter_khz 1000
+reset_config srst_only
+
+# One 128K flash bank
+flash bank pflash.0 kinetis 0x00000000 0x20000 0 4 $_TARGETNAME