Kernel build fix related to external toolchain use

This patch prevents the user from select "linux (Same version as linux
headers)" as a choice for building the kernel when an external binary
toolchain is used, since "same version as linux headers" doesn't make
sense when an external toolchain is used.

It fixes the issue encountered by Hartley <hartleys@visionengravers.com>

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>