Add `vbump` and `release` targets, defaulting to Z

src/mkit.mk

 install: all
 	@$(MKIT_DIR)/make install
 
+release:
+	@$(MKIT_DIR)/make release_z
+
 release_x:
 	@$(MKIT_DIR)/make release_x
 
 uninstall:
 	@$(MKIT_DIR)/make uninstall
 
+vbump:
+	@$(MKIT_DIR)/make vbump_z
+
 vbump_x:
 	@$(MKIT_DIR)/make vbump_x