lldpd: also try /usr/lib/os-release if /etc/os-release is not present

Usually, `/etc/os-release` is a symlink to `/usr/lib/os-release` but on
some of them, programs are expected to fallback to
`/usr/lib/os-release`.
build: hide invocation of sed command to generate some files

Replace them with `AM_V_GEN`.