[meta-xilinx] Dropping 'external-xilinx-toolchain' support from meta-xilinx

[Shaun Purvis]

Only just tuning into the tail end of this thread so I apologize if
this isn't that relevant but...

- Sourcery G++ Lite is now Sourcery CodeBench Lite and available for
download from Mentor Graphics (post-registration)
- The Xilinx version of the toolchain is available for download form
here: http://www.xilinx.com/guest_resources/gnu/

So in theory you could diff the two for file/fodler changes.  But as
far as I know the only significant difference is VFP/NEON support and
some name changes.

I personally like the idea of having the option to use the original
Sourcery toolchain but, as already mentioned, it would only apply to
Zynq products (not MicroBlaze).

Shaun