Add signal source mode to hackrf_transfer

Now hackrf_transfer can act as an analog signal source to generate a
pure SINE wave at the given frequency.
Minor changes to tx_callback

Finalize the error procedure that was accidentally deleted.
Change SGPIO CPLD decimation API interface to be one-based (decimation of 1 to 8).
SGPIO: Add CPLD RX Q channel inversion, API to control.
CPLD: Update bitstream files with RX Q channel flip.
Configure APB3 for appropriate clock sources during start-up.
Add GCC "used" attribute to prevent hard_fault_handler_c() from being removed. It trips up -flto.
Add parentheses to silence ambiguous expression warning.
Remove RF power enable from blinky -- it's not necessary.
Replace GPIO control of 1V8 supply with official function enable_1v8_power().
Add HackRF One RF power enable for samples that RX or TX.
USB0 and USB1 LEDs are only on Jellybean and Jawbreaker.
Do not configure BOOT[3:0] pins in SCU, as they should be idle/input/hi-Z on HackRF.
Merge pull request #118 from dovecho/master

Add signal source mode to hackrf_transfer
Merge pull request #119 from jboone/cpld_q_invert

SGPIO API change and CPLD Q channel invert feature.
Merge pull request #121 from jboone/cleanup_201408

Miscellaneous clean-up
cleaned up signal source mode