[WIP] Fix 1kB ping crash

[szszszsz]

Following patch should fix #67 (crash on 1kB ping message). Changes:

• Do not handle PING as a custom command (only on FINAL stage firmware).
• Add new command for the Python client: ping.

Porting fix from Nitrokey/nitrokey-fido-u2f-firmware, where it was tested. Not tested on U2F-Zero hardware. For solution-related questions please ask @adamglozik directly.

WIP prefix added, since the build (tested on Simplicity Studio v3 / Keil v9.53, ATECC_SETUP_DEVICE not defined) is blocked due to an unrelated compilation error for u2f_printx disabling macro (see TOFIX prefixed commit), which needs correction (ideally by removing TOFIX commit and applying chosen approach with the macros).

Edit 20180709: added missing brace and removed TOFIX commit.

Fixes #67

CC: @adamglozik @jans23

[szszszsz]

I have added missing brace and removed TOFIX commit.