spl: Add generic spl_load function

spl: Add generic spl_load function

Implementers of SPL_LOAD_IMAGE_METHOD have to correctly determine what
type of image is being loaded and then call the appropriate image load
function correctly. This is tricky, because some image load functions
expect the whole image to already be loaded (CONFIG_SPL_LOAD_FIT_FULL),
some will load the image automatically using spl_load_info.read()
(CONFIG_SPL_LOAD_FIT/CONFIG_SPL_LOAD_IMX_CONTAINER), and some just parse
the header and expect the caller to do the actual loading afterwards
(legacy/raw images). Load methods often only support a subset of the
above methods, meaning that not all image types can be used with all
load methods. Further, the code to invoke these functions is
duplicated between different load functions.

To address this problem, this commit introduces a "spl_load" function.
It aims to handle image detection and correct invocation of each of the
parse/load functions.

Although this function generally results in a size reduction with
several users, it tends to bloat boards with only a single user.
This is generally because programmers open-coding the contents of this
function can make optimizations based on the specific loader. For
example, NOR flash is memory-mapped, so it never bothers calling
load->read. The compiler can't really make these optimizations across
translation units. LTO solves this, but it is only available on some
arches. To address this, perform "pseudo-LTO" by inlining spl_load when
there are one or fewer users. At the moment, there are no users, so
define SPL_LOAD_USERS to be 0.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>