cros_sdk: add a mount cache to avoid multiple execs
cros_sdk: add a mount cache to avoid multiple execs

Since we grab a lock before mounting anything, we know that the relevant
list of mounts won't change (since we haven't changed it yet).  So we can
cache the output of `mount` and re-use that in every subsequent check.

BUG=None
TEST=`cros_sdk --enter` still works with simultaneous runs
TEST=`cros_sdk --enter -- ls` still works

Change-Id: I27c98c923761fb777686632fe6e6604ca543cfb6
Reviewed-on: http://gerrit.chromium.org/gerrit/8030
Commit-Ready: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

andrejro/coreos--scripts,andrejro/coreos--scripts,endocode/scripts,BugRoger/scripts,mjg59/scripts,fivethreeo/scripts,BugRoger/scripts,trnubo/scripts,zhang0137/scripts,crawford/scripts,marineam/coreos-scripts,nicescale/scripts,marineam/coreos-scripts,gtank/scripts,mjg59/scripts,vmware/coreos-scripts,coreos/scripts,vmware/coreos-scripts,coreos/scripts,endocode/scripts,sigma/coreos-scripts,nicescale/scripts,nicescale/scripts,zhang0137/scripts,andrejro/coreos--scripts,marineam/coreos-scripts,smilart/scripts,trnubo/scripts,bcwaldon/coreos-scripts,fivethreeo/scripts,zhang0137/scripts,crawford/scripts,gtank/scripts,mischief/scripts,smilart/scripts,gtank/scripts,vmware/coreos-scripts,trnubo/scripts,mischief/scripts,smilart/scripts,mischief/scripts,bcwaldon/coreos-scripts,mjg59/scripts,endocode/scripts,sigma/coreos-scripts,BugRoger/scripts,glevand/coreos--scripts,bcwaldon/coreos-scripts,glevand/coreos--scripts,crawford/scripts,fivethreeo/scripts,sigma/coreos-scripts