Adding E300 Support to master + enhancements for generation 3 products

Contributions from (alphabetically) Ashish Chaudhari, Ben Hilburn, Ian Buckley, Jonathon Pendlum, Josh Blum, Matt Ettus, Moritz Fischer, Philip Balister
Major Changes:
- e300: Initial FPGA code checkin
- x300: Reduced TX SRAM buffer to make meeting timing easier
- b200: Added FE locked sensor
- lib: Enhancements to radio.v and axi_crossbar.v
- lib: Pipelined high-fanout signals in gpio_atr and rx_framer to meet timing
Changed README to markdown. Removed unnecessary README in usrp3.
b200: Pulled in X300 makefile features into B200
b200: Updated coregen project versions to 14.7
e300: Made Makefiles consistent with other gen3 products
e300: Updated coregen project versions to 14.7
usrp3: More Makefile cleanup

- Default targets make sense for each product
- Cleaned up options documentation in make help
usrp3: Moved common radio and cat_cap_gen to lib

- Moved radio.v from X300 to lib/radio. Used by X300 and E300
- Moved catcap, catgen and catcodec from B200 to lib/io_cap_gen. Used by B200 and E300
x300: Fixed odd-length packet drop issue in RX ge_mac

- Xilinx SFP IP asserts error and ~valid for the last byte in odd-length packets
- Patch submitted by Hrishikesh Shelar