readme fix
UHD: Introducing Readme.md in host/
UHD: Updating README in tools/ directory.
uhd tools: removing old impact programmer (it has been renamed)
UHD tools: improving READMEs, fixing licenses.
Merge branch 'origin/uhd/readme_fix' into maint
uhd: updating CHANGELOG for UHD-3.7.1
Updated UHD version to 3.7.1, changed images information accordingly