fwu_metadata: make sure structures are packed

The fwu metadata in the metadata partitions
should/are packed to guarantee that the info is
correct in all platforms. Also the size of them
are used to calculate the crc32 and that is important
to get it right.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>