masterimage.py: fix issue with calling reboot on masterimage/DUT

On systemd systems calling reboot over an ssh connection doesn't
return as expected causing an exception, therefore wrap the call
to reboot in order to avoid this issue.

Also sync the filesystems before rebooting cause otherwise, it will be
done as part of the reboot and could take a very long time and testimage
will fail to access the machine. This issue was observed consistently with
one of our rootfs at Pelagicore.

[YOCTO #11524]

(From OE-Core rev: a9c7f877e5bda32249755dc7014d436e4b85f07a)

Signed-off-by: Erik Botö <erik.boto@pelagicore.com>
Signed-off-by: Stephano Cetola <stephano.cetola@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/lib/oeqa/controllers/masterimage.py

@@ -108,7 +108,7 @@ class MasterImageHardwareTarget(oeqa.targetcontrol.BaseTarget, metaclass=ABCMeta
             time.sleep(10)
             self.power_ctl("cycle")
         else:
-            status, output = conn.run("reboot")
+            status, output = conn.run("sync; { sleep 1; reboot; } > /dev/null &")
             if status != 0:
                 bb.error("Failed rebooting target and no power control command defined. You need to manually reset the device.\n%s" % output)