imagebuilder: complete support for local signing keys

Complete support for local signing keys for APK.

A local key will be always generated, mkndx is always called with
--allow-untrusted as it needs to replace the sign key with the new local
one.

With CONFIG_SIGNATURE_CHECK the local index is signed with the local
key. Local public key is added with the ADD_LOCAL_KEY option.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>

include/rootfs.mk

@@ -47,7 +47,7 @@ apk = \
   IPKG_INSTROOT=$(1) \
   $(FAKEROOT) $(STAGING_DIR_HOST)/bin/apk \
 	--root $(1) \
-	--keys-dir $(TOPDIR) \
+	--keys-dir $(if $(APK_KEYS),$(APK_KEYS),$(TOPDIR)) \
 	--no-cache \
 	--no-logfile \
 	--preserve-env