sdk build: enable nbm signing (if key exists)

sdk/nbproject/platform.properties

@@ -1,4 +1,6 @@
 branding.token=jmonkeyplatform
+keystore=../nbproject/private/keystore.jks
+nbm_alias=jmeupdates
 cluster.path=\
     ${nbplatform.active.dir}/extide:\
     ${nbplatform.active.dir}/harness:\