external-xilinx-toolchain: Fix namespace change
external-xilinx-toolchain: Fix namespace change

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

Signed-off-by: Nathan Rossi <e1e6d173c6873c0ae2b831215d22380d293bb314@xilinx.com>

Xilinx/meta-xilinx,Xilinx/meta-xilinx,nathanrossi/meta-xilinx,Xilinx/meta-xilinx,nathanrossi/meta-xilinx,Xilinx/meta-xilinx,Xilinx/meta-xilinx,nathanrossi/meta-xilinx,nathanrossi/meta-xilinx,Xilinx/meta-xilinx