spl: Add support for passing handoff info to U-Boot proper

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>

include/asm-generic/global_data.h

@@ -125,6 +125,9 @@ typedef struct global_data {
 #if CONFIG_IS_ENABLED(BLOBLIST)
 	struct bloblist_hdr *bloblist;	/* Bloblist information */
 	struct bloblist_hdr *new_bloblist;	/* Relocated blolist info */
+# ifdef CONFIG_SPL
+	struct spl_handoff *spl_handoff;
+# endif
 #endif
 } gd_t;
 #endif