build: Fix crremote()

The script was checking a different value than it was reporting needed
to be set. .username is proper value to check for, so instruct the user
to set that value.

Additionally, when CRUSER is not set properly, the script should not
report that you can "push to crremote". Therefore, set a return value
for the function.

Change-Id: I70ccd82da0de651fe92eab48d4785246a4936569
Merge "build: Fix crremote()" into lollipop