efi_loader: refactor efi_open_protocol

efi_open_protocol was implemented to call a protocol specific open
function to retrieve the protocol interface.

The UEFI specification does not know of such a function.

It is not possible to implement InstallProtocolInterface with the
current design.

With the patch the protocol interface itself is stored in the list
of installed protocols of an efi_object instead of an open function.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
[agraf: fix efi gop support]
Signed-off-by: Alexander Graf <agraf@suse.de>