efi_loader: let the variable driver patch out the runtime

Our variable services are only provided at boottime. Therefore when
leaving boottime the variable function are replaced by dummy functions
returning EFI_UNSUPPORTED. Move this patching of the runtime table to the
variable services implementation. Executed it in ExitBootServices().

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