bitbake: cookerdata: Improve message if BBPATH is unset and bblayers.conf not found

If BBPATH isn't set and bblayers.conf isn't found, improve the message
shown to the user to help their understanding of what the problem might
be.

[YOCTO #3271]

(Bitbake rev: 0e639f5cbc813c8d4719019cfdd4287e9a429610)

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

bitbake/lib/bb/cookerdata.py

@@ -253,7 +253,12 @@ class CookerDataBuilder(object):
             data.delVar('LAYERDIR')
 
         if not data.getVar("BBPATH", True):
-            raise SystemExit("The BBPATH variable is not set")
+            msg = "The BBPATH variable is not set"
+            if not layerconf:
+                msg += (" and bitbake did not find a conf/bblayers.conf file in"
+                        " the expected location.\nMaybe you accidentally"
+                        " invoked bitbake from the wrong directory?")
+            raise SystemExit(msg)
 
         data = parse_config_file(os.path.join("conf", "bitbake.conf"), data)