kbuild: centralize .dts->.dtb rule

All architectures that use cmd_dtc do so in almost the same way. Create a central build rule to avoid duplication. The one difference is that most current uses of dtc build $(obj)/%.dtb from $(src)/dts/%.dts rather than building the .dtb in the same directory as the .dts file. This difference will be eliminated arch-by-arch in future patches. MIPS is the exception here; it already uses the exact same rule as the new common rule, so the duplicate is removed in this patch to avoid any conflict. arch/mips changes courtesy of Ralf Baechle. Update Documentation/kbuild to remove the explicit call to cmd_dtc from the example, now that the rule exists in a centralized location.

SWStephen Warren committed 13 years ago