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>

 This is NOT the official version of dialog.  This version has been

 significantly modified from the original.  It is for use by the Linux

 kernel configuration script.  Please do not bother Savio Lam with 

 questions about this program.