efi_loader: helper function to add EFI object to list

efi_loader: helper function to add EFI object to list

To avoid duplicate coding provide a helper function that
initializes an EFI object and adds it to the EFI object
list.

efi_exit() is the only place where we dereference a handle
to obtain a protocol interface. Add a comment to the function.

Suggested-by: Alexander Graf <[email protected]>
Reviewed-by: Simon Glass <[email protected]>
Signed-off-by: Heinrich Schuchardt <[email protected]>
Signed-off-by: Alexander Graf <[email protected]>