runqemu: Set target to rootfs when target is empty

Fixed:
$ runqemu qemux86-64 core-image-minimal
runqemu - ERROR - IMAGE_LINK_NAME wasn't set to find corresponding .qemuboot.conf file

(From OE-Core rev: 992d6075fbb4e8eb82920a477fcc38b3c0dc8cd5)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/runqemu

@@ -1676,6 +1676,9 @@ to your build configuration.
         if multiconfig:
             multiconfig = "mc:%s" % multiconfig
 
+        if self.rootfs and not target:
+            target = self.rootfs
+
         if mach:
             cmd = 'MACHINE=%s bitbake -e %s %s' % (mach, multiconfig, target)
         else: