fdtget-runtest.sh: use printf instead of /bin/echo -e

Not all /bin/echo implementations support the -e option. Instead, use
printf, which appears to be more widely available than /bin/echo -e.

See commit eaec1dbc "fdtget-runtest.sh: Fix failures when /bin/sh isn't
bash" for history.

I have tested this on Ubuntu 10.04 with /bin/sh pointing to both dash
and bash.

Reported-by: Mike Frysinger <vapier@gentoo.org> # and implemented-by
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>