cros_workon: minimize portageq penalties
cros_workon: minimize portageq penalties

Calling portageq is fairly slow, so only call it when required, and
combine the multiple calls into a single one.  In some cases, we'd
end up calling it multiple times which quickly multiplies the slowness.

BUG=None
TEST=`./cros_workon --board x86-alex start dtc` still works
TEST=`./cros_workon --board x86-alex list` still works and is fast
TEST=`./cros_workon --board x86-alex stop dtc` still works

Change-Id: I6ac6ba283c6529384a7981ad4fffa480bae52234
Reviewed-on: https://gerrit.chromium.org/gerrit/18400
Commit-Ready: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

coreos/dev-util,marineam/coreos-dev-util,mgireesh05/dev-util,coreos/dev-util,coreos/dev-util,marineam/coreos-dev-util,mgireesh05/dev-util,mgireesh05/dev-util