Allow custom path to Xilinx distribution in flash script, and default to the Atlys development board USB IDs for flashing.
Allow custom path to Xilinx distribution in flash script, and default to the Atlys development board USB IDs for flashing.

timvideos/HDMI2USB-jahanzeb-firmware,mithro/HDMI2USB,mithro/HDMI2USB,timvideos/HDMI2USB-jahanzeb-firmware,timvideos/HDMI2USB-jahanzeb-firmware,timvideos/HDMI2USB-jahanzeb-firmware,rohit91/HDMI2USB,mithro/HDMI2USB,rohit91/HDMI2USB,mithro/HDMI2USB,rohit91/HDMI2USB,rohit91/HDMI2USB,rohit91/HDMI2USB,timvideos/HDMI2USB-jahanzeb-firmware,mithro/HDMI2USB