scripts/Makefile.lib: also consider $(CONFIG_SYS_BOARD)-u-boot.dtsi

I have a couple of boards, e.g. foo21, bar33, each with a few
different revisions, so I have

  foo21-revA.dts
  foo21-revB.dts
  bar33-revA.dts
  bar33-revB.dts

Now the necessary U-Boot specific additions for the foo21 boards
doesn't depend on the revision (likely for bar33), so I just want to
have and maintain one

  foo21-u-boot.dtsi

But currently I need to add dummy files foo21-revA-u-boot.dtsi and
foo21-revB-u-boot.dtsi each just containing '#include
foo21-u-boot.dtsi'. And similarly for bar33, and all those files
become quite unwieldy as more revisions need to be supported.

It's quite natural to look for a file named after CONFIG_SYS_BOARD,
with lower precedence of course than a -u-boot.dtsi file with the same
basename as the .dts, but higher than CONFIG_SYS_SOC.

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