Patch ebuild.sh to not nuke the WORKDIR if FEATURES="keepwork".

Patch ebuild.sh to not nuke the WORKDIR if FEATURES="keepwork".

This allows make to do its thing when using Portage in a development
workflow, although the change also exposes how ebuild clean doesn't
clear out the WORKDIR.

TEST=`rm -rf /build/$board/tmp/portage/chromeos-base/kernel-0.0.1 &&
[ -e /build/$board/tmp/portage/chromeos-base/kernel-0.0.1/.unpacked ] &&
rm /build/$board/tmp/portage/chromeos-base/kernel-0.0.1/.unpacked &&
FEATURES=keepwork ebuild-$board kernel-0.0.1.ebuild install`

Then modify some of the source (preferably in the git repository rather
than the WORKDIR).

`[ -e /build/$board/tmp/portage/chromeos-base/kernel-0.0.1/.unpacked ] &&
rm /build/$board/tmp/portage/chromeos-base/kernel-0.0.1/.unpacked &&
FEATURES=keepwork ebuild-$board kernel-0.0.1.ebuild install &&`

On the second ebuild operation, make should be able to tell what
exactly, or if nothing at all, has changed, and rebuild only the
necessary object files.

http://codereview.chromium.org/2922001