Add bootstrapping support

Add tools for launching a root shell on the device, and for sending
serial commands to the device. Extend the make.sh script to push those
and configure a root shell. Commands can now be executed as root via:

adb shell rootshell -c \"touch /tmp/test\"

allowing automatic configuration of the tooling.

rootshell/src/main.rs

@@ -0,0 +1,15 @@
+use std::process::Command;
+use std::os::unix::process::CommandExt;
+use std::env;
+
+fn main() {
+   let mut args = env::args();
+
+   // discard argv[0]
+   let _ = args.next();
+   Command::new("/bin/bash")
+	.args(args)
+	.uid(0)
+	.gid(0)
+	.exec();
+}