[OE-core] Fixing Warnings

[Richard Purdie]

We need to ensure we don't overlap with the warning message fixing work.
I've created:

https://wiki.yoctoproject.org/wiki/Warning_Fixing

which has a list of the remaining warnings I know about and has an
indication of who (if anyone) is addressing them. Please check there
before working on any given warning message.

The good news is we're down to about 115 of them for world assuming the
pending patches merge. 

I'm tempted to turn off the exec_prefix cross linkage warnings in the
release branch for this release since the fixes for those could be too
invasive to take the risk on at this point in the release cycle and most
users probably don't need to see those warnings. Those warnings should
be treated as the lowest priority.

Cheers,

Richard