tests: extract serials to optional local config

Instead of having committed files containing a single developer's serials, use
optional includes to include a local config file for each board.

If you have only a single board connected, simply:

$ make -f Makefile.<board> clean all flash

If you have multiple boards connected, make sure to fill in the appropriate
hla_serial in your openocd.<board>.local.cfg file, and then run the same
commands.

tests/gadget-zero/openocd.common.cfg

@@ -0,0 +1,10 @@
+# Shared openocd script helpers
+
+# put things like "hla_serial 'asdfadfa'" in openocd.<board>.local.cfg to support
+# multiple simultaneously connected boards.
+proc optional_local { LOCAL_FILE } {
+    if { [ file exists $LOCAL_FILE ] } {
+        puts "Loading custom local settings from $LOCAL_FILE"
+        source $LOCAL_FILE
+    }
+}