kernel-yocto: quote kconfig mode check

kernel-yocto: quote kconfig mode check

We allow inheriting recipes to control the kconfig mode used by merge_config.sh
via the KCONFIG_MODE variable. An error crept into the variable reference, and
since it is not quoted, the true condition always runs.

The result is that operations without an explicit kconfig mode cannot trigger
allnoconfig for defconfig builds, which can result in some options being
dropped from the final .config.

Quoting the reference allows it to evaluate properly.

(From OE-Core rev: aad19e4381a8a09c354e5899885997c5b4cd115b)

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>