[mer-bash-setup] Set debian_chroot

Fixes MER#675

Done a more compact way. A more common/sophisticated way would be to
initialize the variable in /etc/bashrc (part of mer-core/setup) from the
content of /etc/debian_chroot fle and add /etc/debian_chroot here.

Signed-off-by: Martin Kampas <martin.kampas@tieto.com>