set_lsb_release.sh: Make it mount rootfs r/w only if necessary.

If we just want to read the current lsb-release, we shouldn't need to break rootfs verification.

Change-Id: I5ba6ddbd9f5801783a568b6806392184b683f628

BUG=none
TEST=none

Review URL: http://codereview.chromium.org/3563001
2 files changed