toolchain/external: sprinkle with some calls to MESSAGE

Thus, the failing step can be easily extracted by autobuilders,
to ease with post-mortem analysis.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>