toolchain/toolchain-external: warn for untested GCC/kernel version

toolchain/toolchain-external: warn for untested GCC/kernel version

The oldest toolchain we test in the autobuilders is the Sourcery ARM
toolchain which is GCC 4.8 and kernel headers 3.13. Therefore, it is
likely that we're missing the required _AT_LEAST dependencies to exclude
packages that don't build with older GCC/headers.

Add a comment to the custom external toolchain that warns when an
untested GCC or kernel headers version is selected.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>