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>

 reporter_name="Yann E. MORIN"

 reporter_url="http://ymorin.is-a-geek.org/"

 reporter_comment="Toolchain for the Raspberry Pi, with hard-float."