scripts: Makefile: Change output overlay from .dtb to .tmp

If the output overlayed file has the .dtb extension, then the build system
may interpret the overlayed file as a SOC DTB file which will result
in the final kernel image including properties from the board files.

Change-Id: I857d5dc731b1bbcd880a7543a437e10f4f9bf912
Signed-off-by: Kyle Yan <kyan@codeaurora.org>