Work around issue in old version of kpartx
Work around issue in old version of kpartx

Older versions of kpartx break if the path is longer than a certain
number of characters and this limit can be hit if the absolute
path of the build directory is too long. This change always passes
relative paths to kpartx so the limit will never be reached.

Xilinx/PYNQ,Xilinx/PYNQ,cathalmccabe/PYNQ,schelleg/PYNQ,cathalmccabe/PYNQ,schelleg/PYNQ,cathalmccabe/PYNQ,yunqu/PYNQ,yunqu/PYNQ,yunqu/PYNQ,Xilinx/PYNQ,cathalmccabe/PYNQ,cathalmccabe/PYNQ,cathalmccabe/PYNQ,yunqu/PYNQ,schelleg/PYNQ,Xilinx/PYNQ,yunqu/PYNQ,schelleg/PYNQ,schelleg/PYNQ,Xilinx/PYNQ