Add option to use ro.serialno for generating device id

On some devices (e.g. ASUS Zenfone) the serial number of the device
is stored in a file in the /factory partition. Consequently, the only
way to load it is using a user space application during the init
process, and to update the ro.serialno system property.

In most places, TWRP already uses the system property to get the
serial number of the device. However, when generating the device ID
used for the backup folder name it checks the boot command line
directly. On these devices, the serialno is not included on the
command line.

Add a TW_USE_SERIALNO_PROPERTY_FOR_DEVICE_ID option that enables
usage of ro.serialno for generating the device id. It is disabled
by default to avoid changing the device id on existing ports.

Change-Id: I4a2eeca883f38fdaeb1209507fd8ebe44b1b04a0

data.cpp

@@ -146,6 +146,16 @@ void DataManager::get_device_id(void) {
 #endif
 
 #ifndef TW_FORCE_CPUINFO_FOR_DEVICE_ID
+#ifdef TW_USE_SERIALNO_PROPERTY_FOR_DEVICE_ID
+	// Check serial number system property
+	if (property_get("ro.serialno", line, "")) {
+		snprintf(device_id, DEVID_MAX, "%s", line);
+		sanitize_device_id(device_id);
+		mConst.SetValue("device_id", device_id);
+		return;
+	}
+#endif
+
 	// Check the cmdline to see if the serial number was supplied
 	fp = fopen("/proc/cmdline", "rt");
 	if (fp != NULL) {