net: shell: Add extra net_pkt RX timing statistics

Add extra net_pkt RX timing information to "net stats" command.
You neeed to enable CONFIG_NET_PKT_RXTIME_STATS and
CONFIG_NET_PKT_RXTIME_STATS_DETAIL config options.
The "net stats" command will print something like this:

Avg RX net_pkt (7) time 459 us	[0->23->16->257->160=456 us]

The extra stats is the end of line inside brackets.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>