buildinstall: Pick correct config with rootfs_size

If there are multiple matching configuration blocks, we should take the
value that is defined in the last one, but only if it actually is there.
That's how it works for other options.

Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

tests/test_buildinstall.py

@@ -251,6 +251,7 @@ class TestBuildinstallPhase(PungiTestCase):
             'release_is_layered': False,
             'buildinstall_method': 'lorax',
             'lorax_options': [
+                ('^.*$', {'*': {}}),
                 ('^Server$', {
                     'x86_64': {
                         'bugurl': 'http://example.com',
@@ -265,6 +266,7 @@ class TestBuildinstallPhase(PungiTestCase):
                 ('^Client$', {
                     '*': {'nomacboot': False}
                 }),
+                ('^.*$', {'*': {}}),
             ]
         })