bitbake: cooker: Fix bbfile_config_priorities when BBFILE_PATTERN is empty

The layer was not in bbfile_config_priorities when BBFILE_PATTERN is empty,
this caused "bitbake-layers show-layers" can't show these layers, this was
incorrect since these layer did exist. Add these layer to
bbfile_config_priorities can fix the problem.

Fixed:
Add BBFILE_PATTERN_core = "" in oe-core/meta/conf/layer.conf
$ bitbake show-layers | grep oe-core

There was nothing, now the layer is shown

(Bitbake rev: 0ff5cdb0cca9266ca29127639494bcfd95e36831)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/cooker.py

@@ -1216,8 +1216,8 @@ class BBCooker:
                     continue
                 elif regex == "":
                     parselog.debug(1, "BBFILE_PATTERN_%s is empty" % c)
+                    cre = re.compile('^NULL$')
                     errors = False
-                    continue
                 else:
                     try:
                         cre = re.compile(regex)