tests: make it more flexible for the ebuild

The current code requires rootdev to be in $PWD which fails when we
build out of tree (like the ebuild).

TEST=`FEATURES=test emerge-link rootdev` passes

Change-Id: I99173ceb557099c20b43f5a31b87107a144efa20
Reviewed-on: https://chromium-review.googlesource.com/234921
Trybot-Ready: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Yunlian Jiang <yunlian@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
1 file changed