build: kernel: avoid unnecessary package rebuilds

The $(LINUX_DIR)/.config timesptamp changes between runs of
make target/compile and make target/install (which builds the image).

Kernel-dependent packages and out of tree modules are built in between
those runs, and they check the .config timestamp to decide if they need
to be rebuilt.

Save the target/compile .config to use its timestamp if the file does
not change between runs.  That way the subsequent kernel packages are
not unnecessarily rebuilt when you run 'make' back to back.

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>