docs: Replace `ping` with `wget`
requested to merge vikas.katariya1/meta-cassini:topics/CAS-75/AMPLATFORMS-3052-part-1 into nanbield-dev
The ping command doesn't exit itself unless the user interrupts,
therefore replace it with wget, which will exit when the page is
downloaded or timeout quickly.
For Corstone-1000, due to its limitation, this could enter into longer timeout, if the network interface is not up at the boot time.
Signed-off-by: Vikas Katariya vikas.katariya@linaro.org