I want to use AES-CBC for secure firmware upgrades on a microcontroller, with AES-CMAC for checking the integrity of the firmware image. The AES-CMAC algorithm needs a AES-128 encryption funtion. For the firmware upgrade, I need decryption only. Thus, I'd like to replace the AES-128 encryption by AES-128 decryption to reduce the program size by omitting the AES encrypt function.
Are there any implications on security if the AES-CMAC algorithm is modified in this way?