efi_loader: improve block device integration with DM

Up to now when devices became available after executing the UEFI
sub-system initialization where not available for EFI applications.

With the patch block devices are added to the UEFI object list whenever
they are probed.

Fixes: f3866909 ("distro_bootcmd: call EFI bootmgr even without having /EFI/boot")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Tested-by: Matwey V. Kornilov <matwey.kornilov@gmail.com>