busybox: support millisecond watchdog periods

The BusyBox watchdog application supports reset periods specified in
milliseconds. The Buildroot package will only allow an integer reset period,
which prevents the use of the required "ms" suffix. Change the watchdog period
configuration item to a string, to allow the use of the "ms" suffix.

Signed-off-by: Simon Dawson <spdawson@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>