scripts/Makefile.lib: Only apply u-boot.dtsi files in the target directory

We only want to apply files such as 'omap5-u-boot.dtsi', which resides
in arch/arm/dts/ to other files in arch/arm/dts/ and not say
test/overlay/.  Rework the make logic to check for -u-boot.dtsi files in
the same directory as their target dts.

Cc: Simon Glass <sjg@chromium.org>
Reported-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Tested-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>