Bluetooth: DIS: integrate app version into FW revision characteristic

Integrated the application version feature of the build system with
the default configuration of the Bluetooth DIS module and its Firmware
Revision characteristic.

The firmware revision string now defaults to APP_VERSION_TWEAK_STRING
if the application version feature is used in a project. This specific
version format is used to unify version formatting with other parts of
Zephyr like the MCUboot module and its versioning Kconfig:
CONFIG_MCUBOOT_IMGTOOL_SIGN_VERSION

Signed-off-by: Kamil Piszczek <Kamil.Piszczek@nordicsemi.no>