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>

doc/develop/uefi/u-boot_on_efi.rst

@@ -265,9 +265,7 @@ This work could be extended in a number of ways:
 
 - Figure out how to solve the interrupt problem
 
-- Add more drivers to the application side (e.g. block devices, USB,
-  environment access). This would mostly be an academic exercise as a strong
-  use case is not readily apparent, but it might be fun.
+- Add more drivers to the application side (e.g.USB, environment access).
 
 - Avoid turning off boot services in the stub. Instead allow U-Boot to make
   use of boot services in case it wants to. It is unclear what it might want