Bug 878079 - Clarify |make install| message when no Android devices are connected. r=ted

DONTBUILD since NPOTB

mobile/android/build.mk

@@ -22,6 +22,11 @@ export ANDROID_SERIAL
 else
 # Determine if there's more than one device connected
 android_devices=$(filter device,$(shell $(ANDROID_PLATFORM_TOOLS)/adb devices))
+ifeq ($(android_devices),)
+install::
+	@echo "No devices are connected.  Connect a device or start an emulator."
+	@exit 1
+else
 ifneq ($(android_devices),device)
 install::
 	@echo "Multiple devices are connected. Define ANDROID_SERIAL to specify the install target."
@@ -29,6 +34,7 @@ install::
 	@exit 1
 endif
 endif
+endif
 
 install::
 	$(ANDROID_PLATFORM_TOOLS)/adb install -r $(DIST)/$(PKG_PATH)$(PKG_BASENAME).apk