Temporary workaround to fix make_chroot --fast

Temporary workaround to fix make_chroot --fast

Three changes here:
  1) parallel_emerge now only disables PORTAGE_LOCKS on board builds. This
     slows down make_chroot --fast, but works around a bug where make_chroot
     --fast sometimes merges broken packages.
  2) Only set PORTAGE_LOCKS if it's not already set. This allows users to
     override PORTAGE_LOCKS and request that it be enabled using
     PORTAGE_LOCKS=true
  3) Only add the no-env-update feature if PORTAGE_LOCKS=false. This feature
     is only needed for that case.

Long term fix is to patch PORTAGE_LOCKS feature to lock postinst. That'll be
tracked in a separate bug.

BUG=chromium-os:6750
TEST=Run make_chroot --fast. It works every time now, but is slower.
     build_packages && build_image run at same speed as before.

Change-Id: I12ab40671034e10cd2ffbba45281ca44718d1d2c

[Nick Sanders, 2010-09-15 22:44:51]

lgtm