[BUG] usage of $kernel_addr_r in distro_efi_boot()

[Heinrich Schuchardt]

If $kernel_addr_r cannot be read, distro_efi_boot() falls back to
address 0. This will lead to a segmentation fault on many systems.

distro_efi_read_bootflow_net() has a fallback to CONFIG_SYS_LOAD_ADDR
and may have loaded to that address.

Shouldn't we be consistent in what we program?

Please, use a field in the bflow variable to indicate if and where an
EFI binary has been loaded.

Best regards

Heinrich