kernel: Make use of KERNEL_MAKE

kernel: Make use of KERNEL_MAKE

Make use of KERNEL_MAKE in kernel packages were easily possible.
This moves some more code to common places and reduces the number of
lines.

It is defined like this:
KERNEL_MAKE = $(MAKE) $(KERNEL_MAKEOPTS)
KERNEL_MAKEOPTS = -C $(LINUX_DIR) $(KERNEL_MAKE_FLAGS)

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>