scripts/Makefile.lib: Always have ...-u-boot.dtsi be able to override

The intention of having a -u-boot.dtsi file is to be able to make
changes to the provided upstream dts files as well as to be able to add
nodes.  Change the logic for adding the file from making it the last
included file at the top of the dts to being included at the end of the
file.

Cc: Jean-Jacques Hiblot <jjhiblot@ti.com>
Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
Tested-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>