docs: added documentation for usage of the Jackson Labx LC_XO GPSDO, used on USRP B2X0 devices
cmake: fixed UHD_IMAGES_DIR behavior to include directories inside specified images directory
Updated images downloader URL
Merge branch 'maint'

Conflicts:
	host/CMakeLists.txt
Updated master images downloader URL

Conflicts:
	host/CMakeLists.txt
docs: added info on libusbx for Windows
Merge branch 'maint'
utils: fix declaration of "env_path_sep" such that it is always initialized before it is used in the "get_env_paths" function, by moving it from the global scope to inside that function.  This change allows UHD_STATIC_BLOCK(load_modules) to work correctly.
Merge pull request #192 from EttusResearch/mdickens/static_load_fix

utils: fix declaration of "env_path_sep" such that it is always initiali...
utils: fix declaration of "env_path_sep" such that it is always initialized before it is used in the "get_env_paths" function, by moving it from the global scope to inside that function.  This change allows UHD_STATIC_BLOCK(load_modules) to work correctly.
Merge branch 'maint'
paths: get_module_paths adds share/uhd/modules to the list of applicable paths
utils: usrp_burn_mb_eeprom can now burn multiple EEPROM values

* example: usrp_burn_mb_eeprom --key="name,ip-addr" --val="my_usrp,192.168.10.3"
* As before, omitting the --val argument will just read off all named keys
* If number of given keys and vals don't match, the util will throw an error
usrp_burn_mb_eeprom: removed unnecessary if-statement and fixed whitespace issue
uhd_images_downloader fixes/improvements

* Permissions checking leads to clearer errors when user doesn't have write permissions
* Relative paths work
uhd: help strings for 'stats' and 'progress' were reversed
uhd_images_downloader: more improvements/fixes

* Better error handling
* Improved default download location logic
BUG #203:  Initialized gain values to 0.0
BUG #183:  B200 High CPU Usage:  Created a single thread to handle libusb events and expanded packet size to 16k
BUG #183: Fixed typo

Signed-off-by: Moritz Fischer <moritz.fischer@ettus.com>