toolchain-external: on ARM, show comment about the availability of Linaro toolchains

On ARM, Linaro external toolchains are only visible if the user
selects Cortex-A8 or Cortex-A9. Therefore, we add a comment that tells
the user that the Linaro toolchains are only available under those
conditions.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>