Major improvements to usrp_n2xx_simple_net_burner

* Improved utility's memory usage
* Moved packet structure and relevant #defines to source file, removed header
* Formalized packet code enums and removed unused code
* Added progress percentage to image writing and verification
* Deprecated --no_fw, --no_fpga, and --auto_reboot options, warning message displays when used
* Other misc. coding improvements
x3x0 docs: the word 'firmware' is misleading
Merge branch 'maint'