Advantages of encrypting a symmetric key with a public key vs using a fixed symmetric key
What is the real advantage of option 2 (asymmetric encryption of firmware)?


As correctly pointed by the question, not much. All I can think of:


It allows to change the symmetric key at each firmware release (actually, the firmware will be encrypted with a random symmetric key, sent asymmetrically encrypted in a header). This advantage is mitigated by the fact that the asymmetric private decryption key can hardly be changed at each release, and must be secret.
The developer/party encrypting the code needs no secret for that. This advantage is mitigated by the fact that the developer/party signing the code needs a private key, which must be secret.