Running a fresh installation of the Broadcom RPI 2 latest, on the Raspberry Pi 3 B, I am receiving the warning about /lib/modules/4.9.80-1-ARCH/modules.devname not found ignoring.
The system still boots fine, because everything is there, but the warning still displays during boot.
I noticed because I am shooting for a "silent" boot and that warning is displayed, even when using the "guiet" option in cmdline.txt.
I don't think it is anything I did, as this is a fresh installation, plus a "pacman -Syu vim"... and I added quiet to /boot/cmdline.txt ... that's all.