vbe: Allocate space for the FIT header

vbe: Allocate space for the FIT header

It is convenient to use TEXT_BASE as a place to hold the FIT header, but
this does not work in VPL, since SDRAM is not inited yet.

Allocate the memory instead. Ensure the size is aligned to the media
block-size so that it can be read in directly. Improve the
error-checking for blk_read() and add some more debugging.

Keep the existing TEXT_BASE mechanism in sandbox to avoid an
'Exec format error' when trying to run the image.

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