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>