[meta-xilinx] tools: useability fixes

[Jean-Francois Dagenais]

Hi guys,

I just started using the meta-xilinx-tools layer and came into some minor 
issues that are easily addressed and will make everyone's live a little  
simpler. Plus a fix related to wic useage (I use it to create an sdcard
image).

Note that since the commit messages are meant to be applied, they are not
formulated as questions and seem definitive. Obviously feel free to comment!