bitbake/cooker: fix idle command processing in servers

bitbake/cooker: fix idle command processing in servers

idle command processing in each of the servers does not handle an explicit
None return value, which means the goggle UI ends up repeatedly adding
"Tasks Summary:" rows to the list.

This patch modifies BBCooker.buildTargets.buildTargetsIdle to return False
when BuildCompleted is fired, as is done in BBCooker.buildFile.buildFileIdle.
It may be that the correct way to fix this is to change the idle command
processing in the servers.

Signed-off-by: Joshua Lock <josh@linux.intel.com>