bloblist: Support allocating the bloblist

bloblist: Support allocating the bloblist

Typically the bloblist is positioned at a fixed address in memory until
relocation. This is convenient when it is set up in SPL or before
relocation.

But for EFI we want to set it up only when U-Boot proper is running. Add
a way to allocate it using malloc() and update the documentation to cover
this aspect of bloblist.

Note there are no tests of this feature at present, nor any direct testing
of bloblist_init().

This can be added, e.g. by making this option controllable at runtime.

Signed-off-by: Simon Glass <sjg@chromium.org>