net: ping: make do_ping() available via <net.h>

net: ping: make do_ping() available via <net.h>

Make the do_ping() function in cmd/net.c a global one by getting rid of
the static qualifier, and move the prototype declaration from net-lwip.h
to net-common.h. This makes the function available to other parts of
U-Boot when CONFIG_NET=y, as was already the case when
CONFIG_NET_LWIP=y.

This is a peparation step to make the sandbox tests use a common API
between NET and NET_LWIP.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>