layer.conf: add back append to BB_HASHBASE_WHITELIST

Add back the append override, as the '+=' operator will make the
default value of BB_HASHBASE_WHITELIST in oe-core not have any
effect.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>

meta-integrity/conf/layer.conf

@@ -24,7 +24,7 @@ LAYERRECOMMENDS_integrity = "\
     tpm \
 "
 
-BB_HASHBASE_WHITELIST += "\
+BB_HASHBASE_WHITELIST:append += "\
     RPM_FSK_PATH \
 "
 

meta-signing-key/conf/layer.conf

@@ -64,7 +64,7 @@ RPM_GPG_PASSPHRASE ??= "SecureCore"
 BOOT_GPG_NAME ??= "SecureBootCore"
 BOOT_GPG_PASSPHRASE ??= "SecureCore"
 
-BB_HASHBASE_WHITELIST += "\
+BB_HASHBASE_WHITELIST:append += "\
     SYSTEM_TRUSTED_KEYS_DIR \
     SECONDARY_TRUSTED_KEYS_DIR \
     MODSIGN_KEYS_DIR \