scripts: check for running as root

Running as root is really, really dangerous.

Add a runtime-check that refuses to build if running as root.
Can be overriden with a double switch in the menuconfig.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

 #!/bin/sh

 set -e

 printf "Running autoconf...\n"

 autoconf -Wall --force

 printf "Done. You may now run:\n    ./configure\n"