Allow overriding ostree-build-yocto and BB_ENV_EXTRAWHITE

If you create a $workdir/overrides/ostree-build-yocto it will be used
instead of the standard ostree-build-yocto. Also, ostree-build-yocto
will append OSTREE_BB_ENV_EXTRAWHITE to the BB_ENV_EXTRAWHITE (that
it sets via oe-init-build-env).

This two changes allow us to use the python27 and git19 scls on rhel6
by having an override file that does something like:

 #!/bin/sh
 OSTREE_BB_ENV_EXTRAWHITE="LD_LIBRARY_PATH" scl enable python27 git19 "/PREFIX/lib/gnome-continuous/ostree-build-yocto $*"

Where PREFIX is where you installed gnome-continuous.