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

scripts/kconfig/Makefile

@@ -11,6 +11,9 @@ else
 Kconfig := Kconfig
 endif
 
+# We need this, in case the user has it in its environment
+unexport CONFIG_
+
 xconfig: $(obj)/qconf
 	$< $(Kconfig)