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