catalyst/targets/generic_target.py: Pass TERM through to the chroot

catalyst/targets/generic_target.py: Pass TERM through to the chroot

Avoid:

  Running stage1-chroot.sh in chroot /var/tmp/catalyst/tmp/default/...
  tput: No value for $TERM and no -T specified

by passing the caller's TERM environment variable [1] through to the
chroot.  If the caller does not supply TERM, default to 'dumb' which
disables color etc., but should be the most portable.  On Gentoo, the
dumb terminfo (/usr/share/terminfo/d/dumb) is distributed as part of
ncurses [2].  You can list supported terminals with toe, which is also
distributed with ncurses [2]:

  $ toe
  ansi            ansi/pc-term compatible with color
  dumb            80-column dumb tty
  linux           linux console
  ...

[1]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03
[2]: http://www.gnu.org/software/ncurses/