Add uhd cmake

I've added in the final tweaks to get this system working.  I'll do the same on the GR side right after this one. These changes will work no matter if "find_package(UHD ...)" is issued from within GR or otherwise, and takes care of the condition that "find_package(gnuradio ...)" is issued -before- the UHD find_package, and thus that GR's FindUHD.cmake is called first. The ENV variables allow FindUHD.cmake to determine that UHDConfig* were actually used, and thus to determine whether to do a check itself (or, not).  What fun! - MLD