recipes-kernel: Move DTB generation to meta-xilinx

oe-core changed the DTB generation to use the linux make system.
http://git.openembedded.org/openembedded-core/commit/?id=72980d5bb465f0640ed451d1ebb9c5d2a210ad0c
This resulted in the kernel make failing as the meta-xilinx contained
dts/dtsi files are not in the kernel tree.
To resolve, move the DTB creation using DTC into meta-xilinx layer.
This will correct the make error and build the DTS files in meta-xilinx.
Additionally the user can set KERNEL_DEVICETREE within local.conf to
build an in-kernel-tree DTS if they desire.

Signed-off-by: Sipke Vriend <sipke.vriend@xilinx.com>
linux : Use overrides to prevent global application of SRC_URI_appends.

Limt the scope of the SRC_URI additions to the machine/soc they apply to. If
this layer is used with other BSP's, the patches may cause problem with
other linux-yocto bbappends.

Signed-off-by: Philip Balister <philip@balister.org>
recipes-kernel:linux-yocto: Move common items to include file.

linux-yocto recipe was modified to use overrides for SRC_URI.
Extend this to linux-yocto-tiny by moving common parts to an include file.
Update external cache variables to use overrides in case other BSP
wish to use an external cache.
Correct typo form microblaze configuration file.

Signed-off-by: Sipke Vriend <sipke.vriend@xilinx.com>