FWU: cmd: Add a command to read FWU metadata

Add a command to read the metadata as specified in the FWU
specification and print the fields of the metadata.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Etienne Carriere <etienne.carriere@linaro.org>