emerge: replace --rebuild with finer grained opts

emerge: replace --rebuild with finer grained opts

Replace --rebuild option with --rebuild-if-* options.

--rebuild-if-new-rev [ y | n ]
  Rebuild packages when dependencies that are used at both build-time and
  run-time are built, if the dependency is not already installed with the
  same version and revision.

--rebuild-if-new-ver [ y | n ]
  Rebuild packages when dependencies that are used at both build-time and
  run-time are built, if the dependency is not already installed with the
  same version. Revision numbers are ignored.

--rebuild-if-unbuilt [ y | n ]
  Rebuild packages when dependencies that are used at both build-time and
  run-time are built.

Change-Id: Ia50c1702bfe1b98a8d1891740e7bbb045921a905

Review URL: http://gerrit.chromium.org/gerrit/280