[oe] gcc recipes cleanup

[Richard Purdie]

Hi,

I've threatened for a long time to clean up the mess the gcc recipes
were in. Over the course of several commits I've tried to do this,
moving code to common .inc files where possible and making it clear
which files affect which types of gcc build (target, cross, sdk,
cross-initial, kernel, native).

Whilst I hope I've not broken anything, the changes are pretty drastic
so please test out your favourite machines/distros/combinations and
report any problems.

I did notice various things which simply couldn't have worked and now
everything is standardised, I'm optimistic that fixes to the toolchain
should be easier to make. Strange side effects from target into cross
and other nasty effects of the old recipes should be much harder to
create now too. meta-toolchain should benefit from the standardised sdk
recipes.

Cheers,

Richard