allow to sign apk using a specific keystore

Makefile

@@ -117,6 +117,10 @@ else
   $(warning Flavor environment variable not set.)
 endif
 
+ifneq ($(KeyStore),)
+  MSBUILD_PARAM += -p:AndroidKeyStore=True -p:AndroidSigningKeyStore="$(KeyStore)" -p:AndroidSigningStorePass=env:MyAndroidSigningStorePass -p:AndroidSigningKeyPass=env:MyAndroidSigningKeyPass
+endif
+
 ifeq ($(detected_OS),Windows)
   define remove
     if exist $(1) ( $(RM) $(1) )