include/kernel: Switch to git download method

Utilize the existing git download logic from include/download.mk and migrate
the kernel download over to it. This avoids repeatedly cloning kernel sources
after a make target/linux/clean for instance.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name> [fix build error]