ti-pru-sw-example: first addition of PRU sw examples recipe

ti-pru-sw-example: first addition of PRU sw examples recipe

* Add PRU sw examples recipe for the omapl138 machines

The ti-pru-sw-example recipe builds example codes that demonstrate use
of the AM180x-evm PRU, compiles the PRU binaries, and saves example
executables and binaries to the filesystem.  The examples included and
manual building instructions are outlined at:
http://processors.wiki.ti.com/index.php/PRU_Linux-based_Example_Code

Signed-off-by: Melissa Watkins <m-watkins@ti.com>
Signed-off-by: Chase Maupin <chase.maupin@ti.com>
Signed-off-by: Koen Kooi <k-kooi@ti.com>