linux-dtb.inc: Fix dtb generation for kernels newer than 3.8

linux-dtb.inc: Fix dtb generation for kernels newer than 3.8

The 3.8 kernel has change the default directory where the dtb file is
stored. The change has been done at:

,----[ Quote of 3.8 kernel change ]
| commit 499cd8298628eeabf0eb5eb6525d4faa0eec80d8
| Author: Grant Likely <grant.likely@secretlab.ca>
| Date:   Tue Nov 27 16:29:11 2012 -0700
|
|     ARM: dt: change .dtb build rules to build in dts directory
|
|     The current rules have the .dtb files build in a different directory
|     from the .dts files. The only reason for this is that it was what
|     PowerPC has done historically. This patch changes ARM to use the generic
|     dtb rule which builds .dtb files in the same directory as the source .dts.
|
|     Cc: Russell King <linux@arm.linux.org.uk>
|     Cc: Arnd Bergmann <arnd@arndb.de>
|     Acked-by: Olof Johansson <olof@lixom.net>
|     Cc: linux-arm-kernel@lists.infradead.org
|     Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
|     [swarren: added rm command for old stale .dtb files]
|     Signed-off-by: Stephen Warren <swarren@nvidia.com>
|     Signed-off-by: Rob Herring <rob.herring@calxeda.com>
`----

This change adds support for both places to backward and forward
compatibility.

(From OE-Core rev: 0ec3710b8dcae311e8d9d676d5f1c6843a81383b)

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>