Match whole words when changing .config options

Make KCONFIG_{ENABLE,DISABLE,SET}_OPT match whole words only.
Fixes that BR_INET_IPV6 enables CONFIG_IPV6 in the kernel's
.config, but also nukes everything with "CONFIG_IPV6" in its name, e.g.
CONFIG_IPV6_PRIVACY.  This in turn causes any IPv6 options in a custom
kernel .config to be lost and oldconfig going interactive.

Signed-off-by: Malte Starostik <m-starostik@versanet.de>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>