cmd: fwu: make changes for supporting FWU metadata version 2

Add support for displaying data specific to FWU metadata version
2. Because the size of the v2 metadata structure is read from the
structure itself, allocate memory for the metadata structure by first
getting the size of the structure.

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