This change will make it easier to manually type cmake commands. The old flags are still supported for now, but a deprecation warning is printed. And if both an old and new flas are set and are not identical, the build with error.
The BUILD_BITCOIN_X flag made sense for some features that result in a bitcoin-x executable being generated, but it wasn't used consistently only for this particular scenario. For instance there is no bitcoin-chronik or bitcoin-zmq executable. Also we use the same flag BUILD_BITCOIN_WALLET both for enabling the wallet feature in bitcoind and for selecting the bitcoin-wallet.
