README: Updated README files for meta-xilinx and BSPs

* Cleaned up some duplication
* Added README for meta-kc705
* Added additional information

Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>
Move include of 'soc-family.inc' into Tune definitions

* Resolves issues with linux-xlnx recipe when building with machines
  that are not using SOC_FAMILY features.

Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>
conf/machine: Remove obsolete machines

* 'zynq' and 'microblazeel' are not real target machines, remove the
  obsolete non-functional machine configurations.

Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>