toolchain-external: reorder the contents of toolchain-external.mk

toolchain-external: reorder the contents of toolchain-external.mk

When the toolchain-external logic will be split into separate packages,
the order in which things are defined in toolchain-external.mk no
makes less sense. So reorder things in a more logical fashion.

Also add a few more comments to the different sections.

No functional changes at all. The output of 'make -qp' hasn't changed,
except for the order of arguments in
TOOLCHAIN_EXTERNAL_TOOLCHAIN_WRAPPER_ARGS.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Romain Naour <romain.naour@gmail.com>
[Arnout: split off into separate patch, slightly change some comments,
 reordered some parts]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>