linux/linux.mk: custom dts path - build dtb only from *.dts files

When building device tree blobs from custom *.dts files, buildroot
initializes KERNEL_DTS_NAME variable from all given file names.
This causes that user can't provide one *.dts file and some other
*.dtsi files as dependencies.

Problem is fixed by adding filter for initializing KERNEL_DTS_NAME
variable with *.dts files only. All user provided files are copied
into kernel source tree, but only file names suffixed with *.dts
are used for building appropriate *.dtb files.

[Thomas: add comment into the code to explain why we are filtering
.dts files only.]

Signed-off-by: Ivo Slanina <ivo.slanina@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>