Sercomm firmwares are encrypted with AES 256 CBC.
The key is generated with a custom algorithm from the firmware tag:
	char key[32];
	char version[32];
	char iv[32];
	char random[32];
	char size[32];
Key must be generated with Sercomm's algorithm. However, the rest of the
header can be empty. IV and random are set to 0 on purpose.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>