TWRP device version string

For building through jenkins. Export a variable to the shell
so that we don't monkey patch variables.h and keep the git tag
clean in the version string.
Jenkins will export the variable TW_DEVICE_VERSION=n where n > 0.
The makefile will use this variable to show the device string
in twrp startup. For mass production builds, omit the
environment variable to default to 0.

Change-Id: I0d6eb764255d7069c0fb4a378522a009cfe4054f
diff --git a/variables.h b/variables.h
index 9cdbfb6..3446902 100644
--- a/variables.h
+++ b/variables.h
@@ -17,7 +17,8 @@
-#define TW_VERSION_STR              "3.0.3-0"
+#define TW_MAIN_VERSION_STR       "3.0.3"
 #define TW_USE_COMPRESSION_VAR      "tw_use_compression"
 #define TW_FILENAME                 "tw_filename"