kernel: prevent additional config from being always rebuilt

 * A simple "make kernel" or incremental system build
   always triggers a complete kernel recompilation,
   even though the kernel sources were unchanged

 * The reason is because .additional_config is always
   considered new and invalidates the .config file,
   forcing the kernel to rebuild itself every time

 * Instead, prevent the /dev/null and KERNEL_OBJ dependencies
   having irrelevant timestamp changes with an empty variable

 * Command "make kernel" takes 3 seconds versus 40s before

Change-Id: I3b583077b7ef5873eebde3e4b0e790f5d7587ed3
Signed-off-by: Adrian DC <radian.dc@gmail.com>