toaster: add started property to Build

toaster: add started property to Build

Add a property to the Build model which records whether
the BuildStarted event has occurred for the build.

The proxy for this event is the presence of variables recorded
against the Build: as the buildinfohelper only saves variables
when the BuildStarted event occurs (as the variables aren't
available on the bitbake server before that point), we can
tell whether BuildStarted has happened by counting Variable
objects on the Build.

This can then be used to determine whether a Build "properly"
started, enabling a different dashboard display (left-hand menu
hidden) if the build didn't record any useful information (e.g.
if it had a bad target).

[YOCTO #8443]

Signed-off-by: Elliot Smith <elliot.smith@intel.com>