efi_loader: efi_load_initrd: provide a memory mapped initrd

efi_loader: efi_load_initrd: provide a memory mapped initrd

U-Boot can pass an initrd to subsequent boot stages via the
EFI_LOAD_FILE2_PROTOCOL. The current implementation only supports
this functionality via the efi boot manager: the initrd is taken
from the load options of the BootCurrent variable. This commit adds
support for registering a memory mapped initrd, e.g. loaded from a
FIT image. For now this new method takes precedence over loading the
initrd from the BootCurrent variable (if both are present) because
the BootCurrent variable is not cleared on exiting the boot manager.

Signed-off-by: Adriano Cordova <adriano.cordova@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>