build: wait with package wipe until the last second

Sometimes signing fails, but then it's hard to recover because
the original package is gone and the new one hasn't been created.

build/common.sh

@@ -633,8 +633,6 @@ bundle_packages()
 
 	REDOS=${@}
 
-	sh ./clean.sh packages
-
 	git_describe ${PORTSDIR}
 
 	# clean up in case of partial run
@@ -691,6 +689,8 @@ bundle_packages()
 	# generate index files
 	pkg repo ${BASEDIR}${PACKAGESDIR}-new/ ${SIGNARGS}
 
+	sh ./clean.sh packages
+
 	REPO_RELEASE="${REPO_VERSION}-${PRODUCT_FLAVOUR}-${PRODUCT_ARCH}"
 	echo -n ">>> Creating package mirror set for ${REPO_RELEASE}... "
 	tar -C ${STAGEDIR}${PACKAGESDIR}-new -cf \