bdinfo: get mac address from environment

Add a new print_eth() function to automate the eth*addr env var acquisition
and display.  Affects all arches.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
CC: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
CC: Scott McNutt <smcnutt@psyent.com>
CC: Shinya Kuribayashi <skuribay@ruby.dti.ne.jp>
CC: Michal Simek <monstr@seznam.cz>
CC: Daniel Hellstrom <daniel@gaisler.com>
CC: Ben Warren <biggerbadderben@gmail.com>