hob: sync after image deploy

Showing "Deploy image successful" after 'dd' returns
may determine the user to disconnect the usb stick even
though the writing operations are not finished.
This patch makes sure that the entire image is deployed
on the usb stick before the user is informed about any result.

[YOCTO #5892]

Signed-off-by: Valentin Popa <valentin.popa@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
runqueue: More carefully handle the sigchld handler

We've noticed hanging processes which appear to be looping around
waitpid. Its possible multiple calls to teardown are causing problem
or in theory multiple registrations (although the code should not
allow that). Regardless, put better guards around signal handler
registration.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>