Certain SOCs may not need much data from VBIOS. Some data like VBIOS
version used will be missed but it doesn't affect functionality. Add a
flag to make VBIOS image optional.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>