kernel: check the right directories for rebuild

kernel: check the right directories for rebuild

Pass KERNEL_FILE_DEPENDS to rdep instead of PKG_FILE_DEPENDS, which is
empty. Also don't pass $(CURDIR) as the directory to timestamp, as it
would also pick up non kernel related changes like image building code.

Should fix kernel being rebuild for unrelated changes, as well as not
being rebuild for changes in target/linux/generic.

Fixes: 22ef1c83b35c ("kernel: make the kernel build auto-clean the build dir like package build")
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>