Add force-mirror FEATURE.

force-mirror: Only fetch files from configured mirrors, ignoring
SRC_URI, except when mirror is in the ebuild(5) RESTRICT variable.

BUG=chromium-os:13221
TEST=Test that ebuilds now fail with FEATURES=force-mirror if files are
not in the configured mirror.

Change-Id: I8484d5af8dff97b431398030b33c024ff1295ba0

Review URL: http://codereview.chromium.org/6677171
(cherry picked from commit 1944622240d87dfa90d88ba19d68f023eed9949f)
3 files changed