armhf libcec does not have RPI support enabled.
The armhf version of this package is not compiled with support for
raspberry-pi.
features reported below should specify RPI
bash-4.3# cat /etc/os-release NAME="Alpine Linux" ID=alpine VERSION_ID=3.6.1 PRETTY_NAME="Alpine Linux v3.6" HOME_URL="http://alpinelinux.org" BUG_REPORT_URL="http://bugs.alpinelinux.org" bash-4.3# cec-client -l libCEC version: 4.0.2, git revision: v3.5.0+4185-e4cc4b7, compiled on Sat Apr 22 00:39:57 UTC 2017 by buildozer
build-3-6-armhf.nethq.org
on Linux 4.4.19 (armv8l), features: P8_USB, DRM, P8_detect, randr
Found devices: NONE@
(from redmine: issue id 7754, created on 2017-08-25)