mynewt: Start watchdog when enable by syscfg

Code implied that WATCHDOG_INTERVAL will enable watchdog
in bootloader however it never did hal_watchdog_init
sets up some watchdog data but for most mcu is does
not start watchdog.

Now hal_watchdog_enable() is called when WATCHDOG_INTERVAL
is set to non zero as git history suggested .

Signed-off-by: Jerzy Kasenberg <jerzy@apache.org>