The error message is coming from theYep, you are right. It looks to me a bit like standard kernel compile process' fault/bug.
/usr/share/initramfs-tools/hook-functions script. You are right that
the script does not look at the config file for
CONFIG_FIRMWARE_IN_KERNEL=y, but I'm not sure if it should or not.
The script uses the command 'modinfo -F firmware tg3' to determine
whether or not it should include firmware images in the initrd. The
modinfo command simply inspects the tg3.ko module binary reports if the
module makes any firmware requests. Obviously this isn't the complete
picture.
Somewhere in this process, the script or modinfo needs to find out if
the kernel has firmware images builtin. How this gets accomplished is
probably best left to the script maintainers.