terminal.bbclass: do not export PS1

terminal.bbclass: do not export PS1

With a complex PS1 setup, PS1 might not have all characters correctly escaped
when terminal.bbclass writes the export. This caused the run.do_terminal.PID to
terminate, making it impossible to use the devshell.

As the spawned shell will parse e.g. .bashrc (or whatever rc-file is being
used), PS1 will be reset in the devshell.

(From OE-Core rev: a5e6926cd409140d16391c72316da00ffbfe5429)

Signed-off-by: Anders Darander <anders@chargestorm.se>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>