Allow arch directory to contain .lds without requiring Makefile

The Makefile for a CPU is in arch/($ARCH)/cpu/$(CPU). We want to support
having an .lds file in arch/$(ARCH)/cpu without requiring an additional
Makefile there. This change makes it clear that we expect a Makefile in
the same directory as the link script except in this case.

Signed-off-by: Simon Glass <sjg@chromium.org>