Kconfig: Add COMPILE_TEST option

Kconfig: Add COMPILE_TEST option

Take the COMPILE_TEST option from the Linux Kernel v6.15 and since the
wording there is OK for us too, use it verbatim. Also update the default
for WERROR to be COMPILE_TEST which again matches the Linux Kernel.

This is the first big step needed to allow for "allyesconfig" to be
possible as it then lets us then disable things that aren't valid for a
compile only test.

Signed-off-by: Tom Rini <trini@konsulko.com>