efi_loader: use event callback for initrd deregistration

Currently efi_initrd_deregister() is called in bootefi.c
when the image started from bootefi command returns.
Since efi_guid_event_group_return_to_efibootmgr event is
implemented, so let's use this event for invoking
initrd deregistration.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Tested-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>