linux: factor dts/dtc/dtb handling into a specific include

linux: factor dts/dtc/dtb handling into a specific include

Fixes [BUGID #610]

The powerpc linux-yocto kernels were not creating dtb images
in the deploy directories. This was due to two problems:

  - the dtb generation rules were not being configured
  - the boards were not specifying a device tree in their config

This change addresses the first point by factoring out the
dtb generation routines into a new include that can be used by
multiple recipes.

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>