At present there is an arch-specific area in the SPL handoff area intended
for use by arch-specific code, but there is no explicit call to fill in
this data. Add a hook for this.

Also use the hook to remove the sandbox-specific test code from
write_spl_handoff().

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

There is some basic informaton that SPL normally wants to pass through to
U-Boot, such as the SDRAM size and bank information.

Mkae use of the new bloblist structure for this. Add a new 'handoff' blob
which is set up in SPL and passed to U-Boot proper. Also adda  test for
sandbox_spl that checks that this works correctly and a new 'sb' command
to show the information passed from SPL.

Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Simon Glass <sjg@chromium.org>