efi_loader: remove NULL entries from runtime detach list

Some entries in the system table are set to NULL in ExitBootServices(). We
had them in the runtime detach list to avoid relocation of NULL. Let's
instead assign the pointers dynamically in efi_initialize_system_table() to
avoid the relocation entry.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>