gps_ctrl.cpp: Partially rewritten, to use caching instead of throwing
away the 'wrong' messages.

The old way to do this was when a sensor was requested, to wait for the
corresponding NMEA message to pass by, throwing away all the 'wrong'
ones in the meantime. However, this leads to problems if the user
requests combinations that are based on the same NMEA string.
To avoid this we now try to cache the strings for a 'freshness' period.
examples & utils: return EXIT_FAILURE and EXIT_SUCCESS respectively
instead of 0 and 1.
Merge branch 'maint'
uhd: removed installation of unit test files
uhd: unit tests generate script w/ env paths
Merge branch 'maint'
wbx v4: fixing the drivers for the new rev of the WBX board
wbx: updating the code to allow full range of the VCO.
Merge branch 'maint'
wbx v4: fixing the drivers for the new rev of the WBX board
wbx: updating the code to allow full range of the VCO.
utils: fixed images downloader Mac incompatibility issue
Merge branch 'maint'
uhd: make stream cmd enums easier on SWIG
utils: fixed Windows images downloader bug
Merge branch 'maint34' into maint
cmake: UHD_RELEASE_MODE is now cached and no longer resets between CMake runs
docs: documentation introduces images-related features introduced in 003.005.000
Merge branch 'maint34' into maint
Merge branch 'maint'