efi_loader: add return to efibootmgr event group

When the image loaded by efibootmgr returns, efibootmgr
needs to clean the resources. Adding the event of returning
to efibootmgr is useful to simplify the implementation.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>