build: force .config update if additional kernel config changes

This patch forces a regeneration of the .config if either the
contents of TARGET_KERNEL_ADDITIONAL_CONFIG changes or the value
of TARGET_KERNEL_ADDITIONAL_CONFIG changes.

OPO-360
Change-Id: Ide28be3fab01b9f9ad72aa9d8706131e2b9ac0c7
(cherry picked from commit 1b6aaec2)