When building a kernel on platforms where the filesytem do
not distingush between upper and lower case the rule:
.S -> .s did not work.
In a normal build this is only used for linker scripts.
So create a separate rule for .lds files, and use generic cpp flags.

Patch from: Dan Aloni <da-x@colinux.org>
Modified to use cpp flags + added documentation.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>