Add core support for a bloblist to convey data from SPL

At present there is no standard way in U-Boot to pass information from SPL
to U-Boot proper. But sometimes SPL wants to convey information to U-Boot
that U-Boot cannot easily figure out. For example, if SPL sets up SDRAM
then it might want to pass the size of SDRAM, or the location of each
bank, to U-Boot proper.

Add a new 'bloblist' feature which provides this. A bloblist is set up in
the first phase of U-Boot that runs (i.e. TPL or SPL). The location of
this info may be in SRAM or CAR (x86 cache-as-RAM) or somewhere else.

Information placed in this region is preserved (with a checksum) through
TPL and SPL and ends up in U-Boot. At this point it is copied into SDRAM
so it can be used after relocation.

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