efi: Locate all block devices in the app

efi: Locate all block devices in the app

When starting the app, locate all block devices and make them available
to U-Boot. This allows listing partitions and accessing files in
filesystems.

EFI also has the concept of 'disks', meaning boot media. For now, this
is not obviously useful in U-Boot, but add code to at least locate these.
This can be expanded later as needed.

We cannot use printf() in the early stub or app since it is not compiled
in

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>