tools: Add mkfwumdata tool for FWU metadata image

Add 'mkfwumdata' tool to generate FWU metadata image for the meta-data
partition to be used in A/B Update imeplementation.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
Signed-off-by: Masami Hiramatsu <masami.hiramatsu@linaro.org>