efi_loader: ensure all block devices are probed

Only probed block devices are available in the UEFI sub-system. Multiple
block devices may be involved in the boot process. So we have to make sure
that all block devices are probed. Another reason is that we store UEFI
variables on the ESP which may be on any block device.

On the sandbox before the patch:

=> efidebug devices
No EFI system partition
Device           Device Path
================ ====================
000000001b027c70 /VenHw(e61d73b9-a384-4acc-aeab-82e828f3628b)
000055d078bc1ae0 /VenHw(e61d73b9-a384-4acc-aeab-82e828f3628b)/Uart(0,0,D,D)
000000001b22e0b0 /VenHw(e61d73b9-a384-4acc-aeab-82e828f3628b)/MAC(020011223344,1)

After the patch:

=> efidebug devices
No EFI system partition
Device           Device Path
================ ====================
000000001b027c70 /VenHw(e61d73b9-a384-4acc-aeab-82e828f3628b)
000055bdac8ddae0 /VenHw(e61d73b9-a384-4acc-aeab-82e828f3628b)/Uart(0,0,D,D)
000000001b230920 /VenHw(e61d73b9-a384-4acc-aeab-82e828f3628b)/SD(2)/SD(0)
000000001b233ac0 /VenHw(e61d73b9-a384-4acc-aeab-82e828f3628b)/SD(1)/SD(1)
000000001b233b80 /VenHw(e61d73b9-a384-4acc-aeab-82e828f3628b)/SD(1)/SD(1)/HD(1,GPT,d0a914ee-a71c-fc1e-73f0-7e302b0e6c20,0x30,0x1)
000000001b234110 /VenHw(e61d73b9-a384-4acc-aeab-82e828f3628b)/SD(1)/SD(1)/HD(2,GPT,9330a0ea-8aff-f67a-294c-fa05d60896c3,0x31,0x1)
000000001b22f0e0 /VenHw(e61d73b9-a384-4acc-aeab-82e828f3628b)/SD(0)/SD(2)
000000001b238df0 /VenHw(e61d73b9-a384-4acc-aeab-82e828f3628b)/MAC(020011223344,1)

Fixes: a9bf024b ("efi_loader: disk: a helper function to create efi_disk objects from udevice")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>