scripts/Makefile.lib: Collate all dtsi files for inclusion

At the time of building a device-tree file, all the *u-boot.dtsi files
are looked for, in a particular order, and the first file found is
included. Then, the list of files specified in the
CONFIG_DEVICE_TREE_INCLUDES symbol are included.

Combine these files that are to be included into a variable, and then
include all these files in one go.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>