Bug 1274497 - Avoid running adb every time we run make on Android builds. r=gps

As seen in bug 1274273, the build system is running adb uselessly.
Moreover, on automation, adb doesn't even run, because it requires a
more recent glibc than available, adding confusing error messages to the
logs.

mobile/android/build.mk

@@ -18,19 +18,22 @@ ifneq ($(ANDROID_SERIAL),)
 export ANDROID_SERIAL
 else
 # Determine if there's more than one device connected
-android_devices=$(filter device,$(shell $(ADB) devices))
-ifeq ($(android_devices),)
-install::
+android_devices=$(words $(filter device,$(shell $(ADB) devices)))
+define no_device
 	@echo 'No devices are connected.  Connect a device or start an emulator.'
 	@exit 1
-else
-ifneq ($(android_devices),device)
-install::
+endef
+define multiple_devices
 	@echo 'Multiple devices are connected. Define ANDROID_SERIAL to specify the install target.'
 	$(ADB) devices
 	@exit 1
-endif
-endif
+endef
+
+install::
+	@# Use foreach to avoid running adb multiple times here
+	$(foreach val,$(android_devices),\
+		$(if $(filter 0,$(val)),$(no_device),\
+			$(if $(filter-out 1,$(val)),$(multiple_devices))))
 endif
 
 install::