emerge: add --rebuild and --norebuild-atoms opts

emerge: add --rebuild and --norebuild-atoms opts

Rebuild when build-time/run-time deps are upgraded.

If pkgA has been updated, and pkgB depends on pkgA at both
build-time and run-time, pkgB needs to be rebuilt. This
feature ensures that all packages are consistent when
dependencies that are used at both runtime and build time
are changed.

This feature only rebuilds packages one layer deep. That
means that if you upgrade libcros, for example, packages
that depend directly on libcros will be rebuilt and
reinstalled, but indirect dependencies will not be rebuilt.

BUG=chromium-os:14296

TEST=Test whether packages rebuilding a bunch of packages.

Change-Id: Idbc0532b4b1de28fd9e5a0abe3b7dbe1a3abd2c8

Review URL: http://codereview.chromium.org/6905107