Add some simple scripts for signing/flashing

These are some simple scripts for signing images (with an example with a
hardcoded path), a fixed root key that matches the one checked into the
code, and a few scripts to use the Segger debugger to flash the images.
diff --git a/scripts/jl.sh b/scripts/jl.sh
new file mode 100755
index 0000000..260206d
--- /dev/null
+++ b/scripts/jl.sh
@@ -0,0 +1,5 @@
+#!/bin/bash
+
+source $(dirname $0)/../target.sh
+
+JLinkExe -speed auto -si SWD -device $SOC