I don't know if this works from the boot loader prompt, but you can try to delete the auth key using this note fromTSB16495
http://kb.juniper.net/InfoCenter/index?page=content&id=TSB16495
On SSG 20/140/320M/350M/520/520M/550/550M :
It is not required to update the current boot loader because the integrity check of the boot loader is only done during the installation of a boot loader. During boot-up of the device there is no integrity check done for the boot loader using the image key. Therefore the existing boot loader on the SSG device will keep working correctly after updating the image key on the device.
NOTE: If the old image key is deleted using CLI delete crypto auth-key command, the device skips the integrity check of the ScreenOS firmware while the device boots up. You will see the Ignore image authentication! message on the console while the device boots up.