bitbake: runbuilds: process builds on SIGUSR1

bitbake: runbuilds: process builds on SIGUSR1

Run main processing function 'runbuild' only if SIGUSR1 is
received. This signal is sent by Toaster when build status
is changed (either started, cancelled or finished).

This should stop continuous database polling as run_builds function
will be called only when needed, i.e. after build status is changed.

[YOCTO #8918]

(Bitbake rev: 62d598cc5aa01d23f1e9284e9e926bd55b1d1878)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Michael Wood <michael.g.wood@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>