Move environment.bz2 extraction from ebuild.sh to doebuild() on

Move environment.bz2 extraction from ebuild.sh to doebuild() on
the python side. The python will be able to use it's awareness
of the ${T}/environment to decide what type of ebuild environment
should be generated. For example, if the ebuild environment
should be able to unset variables that have been inherited from
the calling environment, the existence of ${T}/environment
will indicate that the ebuild environment should be isolated from
the calling environment.

svn path=/main/trunk/; revision=8753