This commite eliminates the ping command from NSH and replaces it with a ping...

This commite eliminates the ping command from NSH and replaces it with a ping 'built-in' at apps/system/ping.  The original NSH version of ping violated the portable POSIX interface and, hence, had to be removed.  The new system/ping command uses the new IPPROTO_ICMP AF_INET datagram sockets to implement ping.

Squashed commit of the following:

    apps/system/ping:  Fix some timing issues.
    apps/system/ping:  A few timing related corrections.
    apps/nshlib:  Remove support for the NSH 'ping' command.  The implementation of that command violated the portable POSIX interface and has been replaced with a ping 'built-in' command at apps/system/ping.
    apps/system/ping:  Add larger payload.  Verify content of echoed payload.
    apps/system/ping:  Various fixes for a clean compile
    system/ping: Add new build structure for system ping command.