scripts/runqemu: provide better error message on runqemu ifup fail

If runqemu-ifup fails hen running testimage, a rather cryptic error
regarding "no tty present" is displayed. If this step fails, we
should at least point the user at runqemu-gen-tapdevs. A quick search
of this term in the manual will lead them to "Enabling Runtime Tests
on QEMU" which should give them all the info they need.

(From OE-Core rev: 3b6494fad2b8b65e0d52cda0cdf500e93c72823a)

Signed-off-by: Stephano Cetola <stephano.cetola@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/runqemu

@@ -786,6 +786,9 @@ class BaseConfig(object):
             self.cleantap = True
             logger.info('Created tap: %s' % tap)
 
+        if not tap:
+            logger.error("Failed to setup tap device. Run runqemu-gen-tapdevs to manually create.")
+            return 1
         self.tap = tap
         n0 = tap[3:]
         n1 = int(n0) * 2 + 1