The mask is set only if it's not set already. Also, it picks up the default
install mask from common.sh.
BUG=none
TEST=gmerged chromeos-chrome with PKG_INSTALL_MASK set/unset.
Review URL: http://codereview.chromium.org/2907005
TEST=Tested this by running it locally on my workstation and calling both stateful_update and memento_updater on a ToT build of chromiumOS on an eee1008HA.
Review URL: http://codereview.chromium.org/2283003
Add restart_in_chroot_if_needed to common.sh, and modify the build scripts which referred to assert_inside_chroot to use it instead. The effect is that you don't ever have to explicitly enter_chroot.sh to build (still can, it work's fine).
Update mod_image_for_test.sh to use restart_in_chroot_if_needed
Review URL: http://codereview.chromium.org/1736025
