scripts/kconfig: ensure we use proper CONFIG_ prefix

Now that we get the CONFIG_ prefix from the environment, we must
ensure we use the proper prefix in case the user has it set in
the environment.

Simply unexport CONFIG_ to fallback to our hard-coded default.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Michal Marek <mmarek@suse.cz>