Get Master Private Key from BIP39 Seed
Here are the mechanics for "Mastering Bitcoin" Creating an HD Wallet from the Seed using  bitcoin-explorer and this Python HMAC command.  

Root Seed:

% echo "army van defense carry jealous true garbage claim echo media make crunch" | bx mnemonic-to-seed -p ""

5b56c417303faa3fcba7e57400e120a0ca83ec5a4fc9ffba757fbe63fbd77a89a1a3be4c67196f57c39a88b76373733891bfaba16ed27a813ceed498804c0570

Master Private Key:

% echo 5b56c417303faa3fcba7e57400e120a0ca83ec5a4fc9ffba757fbe63fbd77a89a1a3be4c67196f57c39a88b76373733891bfaba16ed27a813ceed498804c0570 | bx base16-decode | hmac --algorithm sha512 --key "Bitcoin seed" - | cut -c 1-64

b2a0d576b828b537688b561f2cfa8dac3602d54c62bde619ad5331e6c235ee26   

Master Public Key:

% echo 5b56c417303faa3fcba7e57400e120a0ca83ec5a4fc9ffba757fbe63fbd77a89a1a3be4c67196f57c39a88b76373733891bfaba16ed27a813ceed498804c0570 | bx base16-decode | hmac --algorithm sha512 --key "Bitcoin seed" - | cut -c 1-64 | bx ec-to-public

03ca72b45eede592f059b7eaf3da13eb7d8d15aa472b6f79f74820bb22ff596186

Chain Code:

% echo 5b56c417303faa3fcba7e57400e120a0ca83ec5a4fc9ffba757fbe63fbd77a89a1a3be4c67196f57c39a88b76373733891bfaba16ed27a813ceed498804c0570 | bx base16-decode | hmac --algorithm sha512 --key "Bitcoin seed" - | cut -c 65-128

b70d675323c40ec461e0a6af603b1f135fb2af9ae753eeff18922732a73b0f05