skeleton-init-common: improve /etc/hosts generation

If BR2_TARGET_GENERIC_HOSTNAME contains a FQDN, strip the host part and
add it as an alias, e.g.

    127.0.1.1	hostname.example.com hostname

Signed-off-by: Carlos Santos <casantos@datacom.com.br>
Reviewed-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>