libbladeRF: Support for legacy config interface.

Added checks for the legacy config interface being interface 0 instead
of interface 3 which changed in the v1.4 bump.
libbladeRF: Fixed vendor command to enable RF.
libbladeRF: Moved where legacy warning is printed.
libbladeRF: More backward compatibility fixes.
libbladeRF: copy lusb_flash_read() and lusb_flash_write() functions for legacy

This commit copies lusb_flash_read() and lusb_flash_write() from the before
57116c91, thus allowing devices running <= v1.3 to upgrade properly.