kernel: fix build error for external kernel.

fixed build error when external kernel is selected from menuconfig.
The patches present in target/linux/generic does not gets applied
to external kernel and build fails while compiling mac82011 &
regmap-core kernel modules. as a fix added check in Makefile for
CONFIG_EXTERNAL_KERNEL_TREE present or not.

Signed-off-by: Sandeep Sheriker Mallikarjun <sandeepsheriker.mallikarjun@microchip.com>