Update CUSTOM_LUN_FILE from AOSP USB config

AOSP has used a standard USB config for the last 3 years.
Time to update TWRP for devices that still use UMS.

Reference: https://android.googlesource.com/platform/system/core/+/master/rootdir/init.usb.rc

Change-Id: I59690f955a7d6db9ae9d607d46299652b52fe8b8
diff --git a/variables.h b/variables.h
index d5df8d4..c20eb2b 100644
--- a/variables.h
+++ b/variables.h
@@ -170,7 +170,7 @@
 //#define MAX_ARCHIVE_SIZE 52428800LLU // 50MB split for testing
-#define CUSTOM_LUN_FILE "/sys/devices/platform/usb_mass_storage/lun%d/file"
+#define CUSTOM_LUN_FILE "/sys/class/android_usb/android0/f_mass_storage/lun%d/file"