Currently the shell provided via the 'shell' command is always '/bin/sh'
and does not have a TERM variable set. This is a rather spartan shell
experience.
With this patch we add the TERM variable to the whitelist, so that
pagers work better and made the shell binary dependent from the SHELL
environment variable. If no such variable is set, we fall back to using
'/bin/sh'.
Signed-off-by: Claudius Heine <ch@denx.de>
