tools: kwboot: unbreak terminal-only mode

tools: kwboot: unbreak terminal-only mode

Commit 84899e2 ("tools/kwboot: Sync with latest barebox version to
support Armada XP") accidently broke the terminal-only mode (-t) by
removing the test on the bootmsg. Thus even when trying to use kwboot
as a plain terminal, it asks to reboot the target.

This commit simply reintroduces the lost test so that it is possible
again to use kwboot to attach to the target system's console.

Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Stefan Roese <sr@denx.de>