Running `hackrf_cpldjtag` should warn that it is no longer required

[martinling]

The documentation notes that running hackrf_cpldjtag to update the CPLD is no longer required since the 2021.03.1 release. However if someone does want to run a custom CPLD bitstream and runs hackrf_cpldjtag to flash it, there is no warning that the bitstream will be silently ignored. This came up in #1140.

We could perhaps remove the hackrf_cpldjtag utility entirely, but it may still be wanted for working with older or customised firmware versions. If it's going to remain though, then it should probably at least print a message to warn that the standard firmware will now override the bitstream written with it.