V0.54: OTP support improvements

This firmware requires Nitrokey App v1.4 to fully operate. Otherwise the OTP slot writing and configuration update will not work.

In this update mostly OTP features are improved. Namely:

• 320-bit OTP secret #23;
• 64-bit HOTP counter;
• OS startup verification with https://github.com/Nitrokey/nitrokey-hotp-verification - #71 (tested; requires support client side);
• TOTP year 2038+ support.

Known limitations:

• Write to the 15th TOTP slot is disabled (until the next firmware) due to a bug - #91;
• Write to OTP slots in Nitrokey Apps v1.3.2 and older is silently (without a warning to user) disabled due to a modified authorization method (#22). At the moment only Nitrokey App v1.4 can handle it (will be released in coming days). Other features works as usual.

Tested with libnitrokey Nitrokey/libnitrokey@a8028a7 (with TOTP slot count set to 14).

Firmware is signed with szczepan@nitrokey.com, key id:
868184069239FF65DE0BCD7D D9BAE35991DE5B22
(valid until 2020-01-11). ID can be confirmed to be the same with one on the main download site - https://www.nitrokey.com/download.