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.