kernel: Rework kernel make flag to variable mappings

In 2017 we added changes to pass the BUILD_CFLAGS into the kernel
via BUILD_CC. This isn't really correct and the upstream kernel now has
places to pass build cflags, ldflags and more. Update our kernel
make flags to correctly use the kernel's variables. This addresses
concerns raised by kernel developers.

If this breaks some usecase please report it so we can work out how
to fix it properly.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>