tools: mkfwumdata: add support for metadata version 2

Add support for generating the FWU metadata version 2. The tool now
requires the version to be provided as a command-line option. Make
corresponding changes to the tool's manpage.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Tested-by: Michal Simek <michal.simek@amd.com>