yocto-check-layer: Ensure we use OEBasicHash as the signature handler

The layer checks are designed to work with OEBasicHash so ensure that handler
is in use rather than the new hash equivalency one as an example.

(From OE-Core rev: a10bf92516a4771e2dc49ba9f74323d7a87a1619)

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

scripts/lib/checklayer/__init__.py

@@ -245,7 +245,7 @@ def get_signatures(builddir, failsafe=False, machine=None):
     sigs = {}
     tune2tasks = {}
 
-    cmd = ''
+    cmd = 'BB_ENV_EXTRAWHITE="$BB_ENV_EXTRAWHITE BB_SIGNATURE_HANDLER" BB_SIGNATURE_HANDLER="OEBasicHash" '
     if machine:
         cmd += 'MACHINE=%s ' % machine
     cmd += 'bitbake '