cros_sdk: use enter_chroot from sdk_lib/ instead of scripts
cros_sdk: use enter_chroot from sdk_lib/ instead of scripts

BUG=chromium-os:18750
TEST=cros_sdk --enter

Change-Id: Ie73b129b10af0191cbe9bdee9c57d86083e3f041
Reviewed-on: http://gerrit.chromium.org/gerrit/5911
Reviewed-by: Chris Sosa <959d3d631fb6f17f1b07f751372041a1edb210d1@chromium.org>
Reviewed-by: Richard Barnette <9e7d66387742bf40b0d2548ef2b93e7c522ca0eb@chromium.org>
Reviewed-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Reviewed-by: Anush Elangovan <be3ea4b32dfea400cfb02e20cf45a400ef235334@chromium.org>
Tested-by: Zdenek Behan <483cf2a9a4e071d4fd973db6a5a78c40f490667c@chromium.org>

chadversary/chromiumos.chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,coreos/chromite,zhang0137/chromite,zhang0137/chromite,coreos/chromite