arm: Warn that starting with v2018.01 gcc-6 or later is required

There are more and more cases where if we do not use gcc-6.0 or later we
run into problems where our binaries are too large for the targets.
Given the prevalence of gcc-6.0 or later toolchains at this point in
time, we give notice now that starting with v2018.01 we will require
gcc-6 (or later) for ARM.

Signed-off-by: Tom Rini <trini@konsulko.com>