toolchain-external: move wrapper to toolchain directory

toolchain-external: move wrapper to toolchain directory

The toolchain wrapper will be reused for the internal toolchain, so it
belongs in the toolchain directory. Also, the ext- prefix is removed
from it. The build commands are moved to a new toolchain-wrapper.mk.

The wrapper arguments that are also relevant for the internal toolchain
wrapper are moved to toolchain-wrapper.mk, the rest stays in
toolchain-external.mk.

While we're at it, move the building of the toolchain wrapper to the
build step of toolchain-external. There is no specific reason to do
this, other than that it fits better semantically. Also remove the
MESSAGE call, otherwise we'd see:
>>> toolchain-external undefined Building
>>> toolchain-external undefined Building toolchain wrapper
/usr/bin/gcc ...
Having an extra "Building toolchain wrapper' message is pointless.

The useless condition on $(BR2_TARGET_OPTIMIZATION) is removed. It was
always true because it wasn't qstrip'ped first, so clearly it works
without that condition as well.

Also rewrapped some comments and removed the 'external' reference.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Fabio Porcedda <fabio.porcedda@gmail.com>
Cc: Jérôme Oufella <jerome.oufella@savoirfairelinux.com>
Reviewed-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>