OnlyKey can't handle some subkey types

[SkyKwas]

Problem

The OnlyKey app cannot add private keys with specific subkey types or if the primary key is anything other than cert and sign, [CS].

Description

When I create a default key with GnuPG (primary key [CS] and one subkey [E]), the OnlyKey app can read the exported private key and I'm able to select which key (primary or specific sub) I want for a specific slot (ECC1, ECC2, etc.). However, when making custom keys, I noticed a few configurations that the app can't handle:

• The primary key is only for certification, [C].
• A subkey is only for signatures, [S].
• A subkey is only for authentication, [A].

How to reproduce

1. Use the OnlyKey app v5.5.0.

2. Flash firmware v3.0.4-prodc onto an OnlyKey.

3. Make a new key in GnuPG with the following specifications:

• Primary key is only for certification, [C].
• Subkey 1 is only for signatures, [S].
• Subkey 2 is only for encryption, [E].
• Subkey 3 is only for authentication, [A].

4. Export the private key from GnuPG.

5. Try to add the private key using the OnlyKey app under the "Keys" tab; OnlyKey will fail to recognize the key.