kbuild: Allow DTB overlays to built from .dtso named source files

kbuild: Allow DTB overlays to built from .dtso named source files

[Linux commit 363547d2191c]

Currently DTB Overlays (.dtbo) are build from source files with the same
extension (.dts) as the base DTs (.dtb). This may become confusing and
even lead to wrong results. For example, a composite DTB (created from a
base DTB and a set of overlays) might have the same name as one of the
overlays that create it.

Different files should be generated from differently named sources.
 .dtb  <-> .dts
 .dtbo <-> .dtso

We do not remove the ability to compile DTBO files from .dts files here,
only add a new rule allowing the .dtso file name. The current .dts named
overlays can be renamed with time. After all have been renamed we can
remove the other rule.

[Import notes: Adapt to U-Boot by using the cmd_dtco function instead
of cmd_dtc just like the current .dts -> .dtbo rule.]

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Simon Glass <sjg@chromium.org>