Merging the new yocto 1.7 rebase

[Alexander Larsson <alexl redhat com>]

I have rebased gnome-continuous to the latest yocto 1.7 master
(scheduled release date: Oct. 31, 2014). The yocto changes are at:
https://github.com/alexlarsson/poky/tree/gnomeostree-3.14-dizzy

And the gnome-continiuous changes are at:
https://git.gnome.org/browse/gnome-continuous/log/?h=wip/yocto1.7

This builds and smoketests locally for me on F21.

I think now is a good time to do the rebase, as the 3.14 release is out.
However, to get the new yocto to build on RHEL6 i had to make some
changes:

First of all I installed the "python27" and "git19" SCLs. Then I had to
make ostbuild pick these up. This was a bit complex as it normally
cleans the environment when spawning yocto.
What i did was allow overriding ostree-build-yocto as per:
https://git.gnome.org/browse/gnome-continuous/commit/?h=wip/yocto1.7&id=2a2d114a809feb3efef00a90562d1d5544fce3d2

So, in the overrides directory in the build tree i put a file called
"ostree-build-yocto" containing:

 #!/bin/sh
 scl enable python27 git19 "/data/gnome/root/lib64/gnome-continuous/ostree-build-yocto $*"

(where /data/gnome/root is my gnome-continuous prefix)

Of course, this still didn't work, because LD_LIBRARY_PATH is stripped
by bitbake. I tried to work around this by using BB_ENV_EXTRAWHITE, but
several build recipies failed anyway since they manually clean the env.

Instead i used patchelf
(http://nixos.org/releases/patchelf/patchelf-0.8/) like this:
 patchelf --set-rpath /opt/rh/python27/root/usr/lib64 /opt/rh/python27/root/usr/bin/python2.7

which fixed the build

I also rpmbuild --rebuild the tar from F21, as yocto needed a later
version:
https://kojipkgs.fedoraproject.org//packages/tar/1.27.1/7.fc21/src/tar-1.27.1-7.fc21.src.rpm

With the above i managed to build everything up to builddisks, where i
got into issues with guestfs when building on a virtual machine. I think
these problems will not happen on the real machine, as i don't think
this part will be affected by the yocto rebase.