Bug 755206 - Use ANDROID_SERIAL for make install when defined. r=mfinkle

mobile/android/build.mk

@@ -76,22 +76,23 @@ fast-package:
 	@$(MAKE) package MOZ_FAST_PACKAGE=1
 
 ifeq ($(OS_TARGET),Android)
-ifeq ($(MOZ_ANDROID_INSTALL_TARGET),)
+ifneq ($(MOZ_ANDROID_INSTALL_TARGET),)
+ANDROID_SERIAL = $(MOZ_ANDROID_INSTALL_TARGET)
+endif
+ifeq ($(ANDROID_SERIAL),)
 # Determine if there's more than one device connected
 android_devices=$(filter device,$(shell $(ANDROID_PLATFORM_TOOLS)/adb devices))
 ifneq ($(android_devices),device)
 install::
-	@echo "Multiple devices are connected. Define MOZ_ANDROID_INSTALL_TARGET to specify the install target."
+	@echo "Multiple devices are connected. Define ANDROID_SERIAL to specify the install target."
 	$(ANDROID_PLATFORM_TOOLS)/adb devices
 	@exit 1
-else
+endif
+endif
+
+export ANDROID_SERIAL
 install::
 	$(ANDROID_PLATFORM_TOOLS)/adb install -r $(DIST)/$(PKG_PATH)$(PKG_BASENAME).apk
-endif
-else
-install::
-	$(ANDROID_PLATFORM_TOOLS)/adb -s $(MOZ_ANDROID_INSTALL_TARGET) install -r $(DIST)/$(PKG_PATH)$(PKG_BASENAME).apk
-endif
 else
 	@echo "Mobile can't be installed directly."
 	@exit 1