mathworks/socfpga: use sockit DTS
mathworks/socfpga: rebuild CycV dev kit SPL
mathworks/zynq: update ZedBoard clock init to match other boards
mathworks: add clean dl option to build.sh

build.sh now accepts a --cleandl or -c option to clean the ./dl
directory before building a target. This forces buildroot to go
and fetch the latest source from any git repositories when using
a branch (as opposed to a tag or commit) to specify the version