docs(fwu_metadata): add FWU metadata build options
docs(fwu_metadata): add FWU metadata build options

Added the build options used in defining the firmware update metadata
structure.

Change-Id: Idd40ea629e643e775083f283b75c80f6c026b127
Signed-off-by: Manish V Badarkhe <d2342ad575f6bc1a5f5a5b89d6db0f72dd7e0e4c@arm.com>

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware