external-xilinx-toolchain: Fix namespace change

* Fix bb.mkdirhier -> bb.utils.mkdirhier due to namespace change

Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>
external-xilinx-toolchain: Update license checksums

* Remove ${COREBASE}/LICENSE, as this layer and recipe are only licensed
  under MIT

Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>
README: Add note about known issue with XSDK 2013.3 toolchain

Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>
shadow: Add workaround for build issue

* Apply a bbappend for shadow to resolve the known issue
* Remove this change when the issue has been resolved in gcc/binutils

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