image-buildinfo,oeqa/selftest/containerimage: Ensure image-buildinfo doesn't break tests

Having image-buildinfo enabled causes containerimage.ContainerImageTests.test_expected_files
to fail due to the presence of an unexpected file:
  ['./',
   './etc/',
-  './etc/build',
   './etc/default/',
   './etc/default/postinst',

Tweak the class to allow it to be disabled and disable it from the test just in
case it was enabled.

(From OE-Core rev: af67bf422a4df5b7e07894512ff73a5f493682ab)

(From OE-Core rev: f49ab8b1610c045acaed7b964d12f07f969df856)

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

meta/classes/image-buildinfo.bbclass

@@ -64,6 +64,8 @@ def buildinfo_target(d):
 
 # Write build information to target filesystem
 python buildinfo () {
+    if not d.getVar('IMAGE_BUILDINFO_FILE'):
+        return
     with open(d.expand('${IMAGE_ROOTFS}${IMAGE_BUILDINFO_FILE}'), 'w') as build:
         build.writelines((
             '''-----------------------