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>
diff --git a/scripts/kconfig/Makefile b/scripts/kconfig/Makefile
index 3091794..231b475 100644
--- a/scripts/kconfig/Makefile
+++ b/scripts/kconfig/Makefile
@@ -11,6 +11,9 @@
 Kconfig := Kconfig
 endif
 
+# We need this, in case the user has it in its environment
+unexport CONFIG_
+
 xconfig: $(obj)/qconf
 	$< $(Kconfig)