Drop the special am335x_boneblack_vboot target

Drop the special am335x_boneblack_vboot target

Now that am335x_evm boots OK on the Beaglebone black, drop the latter
and update the docs to cover the change.

Also add a few updates about 'make fit' and drop the note about the
security review, as U-Boot's verified boot has had quite extensive
review now.

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