frdm-common: Add OpenOCD configurations for Kinetis L devices

boards/common/frdm/dist/openocd-klx.cfg

+#
+# NXP Kinetis Freedom developer board
+#
+# OpenSDA is the on-board debugger, some boards have a CMSIS-DAP compatible
+# interface, other boards comes pre-flashed with a Segger J-Link compatible
+# firmware. The OpenSDA controller can be re-flashed to provide either of the two.
+# Both interfaces work with OpenOCD, but we need to tell which one we have on
+# our debugger.
+
+# CMSIS-DAP (DAPLink) compatible OpenSDA firmware binary images can be found at:
+# http://www.nxp.com/opensda
+
+# Kinetis L only supports SWD
+transport select swd
+
+# Kinetis L series CPUs
+source [find target/klx.cfg]
+
+reset_config srst_only
+
+$_TARGETNAME configure -event gdb-attach {
+  halt
+}
+$_TARGETNAME configure -rtos auto