kconfig: allow overriding symbols

Allow config variables in .config to override earlier ones in the same
file. In other words,

	# CONFIG_SECURITY is not defined
	CONFIG_SECURITY=y

will activate it. This makes it a bit easier to do

cat original-config myconfig myconfig2 ... >.config;
and run *config as expected.

Signed-off-by: Jan Engelhardt <jengelh@computergmbh.de>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Roman Zippel <zippel@linux-m68k.org>
Cc: Randy Dunlap <randy.dunlap@oracle.com>