toaster: add two-stage commit startup logic

Toaster start script lunches multiple process components
of the toaster system.

This patch adds logic into the startup script to
safely fail startup and do proper cleanup on any error
that may happen during system start.

Bitbake needs to return 0 if it will successfully lunches
the server-mode.

Signed-off-by: Alexandru DAMIAN <alexandru.damian@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>