Log discovery information for debug

For debug purposes it is handy to log all the discovered information
into a human readable log file.

The implementation here logs the information to /var/log/onie.log (the
usual ONIE log file) using the 'logger' command with the 'discover'
tag.

The output looks like:

Nov 20 02:15:13 ogre local0.info discover: onie_neighs = 10.42.0.1,fe80::4638:39ff:fe00:1ab9-eth0,fe80::4638:39ff:fe00:3d86-eth0,fe80::206:67ff:fe24:679b-eth0,
Nov 20 02:15:13 ogre local0.info discover: onie_disco_opt58 = 00005460
Nov 20 02:15:13 ogre local0.info discover: onie_disco_router = 10.42.0.1
Nov 20 02:15:13 ogre local0.info discover: onie_disco_subnet = 255.255.255.0
Nov 20 02:15:13 ogre local0.info discover: onie_disco_opt59 = 000093a8
Nov 20 02:15:13 ogre local0.info discover: onie_disco_domain = moon.com
Nov 20 02:15:13 ogre local0.info discover: onie_disco_interface = eth0
Nov 20 02:15:13 ogre local0.info discover: onie_disco_dns = 10.42.0.1
Nov 20 02:15:13 ogre local0.info discover: onie_disco_siaddr = 10.42.0.1
Nov 20 02:15:13 ogre local0.info discover: onie_disco_serverid = 10.42.0.1
Nov 20 02:15:13 ogre local0.info discover: onie_disco_broadcast = 10.42.0.255
Nov 20 02:15:13 ogre local0.info discover: onie_disco_ip = 10.42.0.10
Nov 20 02:15:13 ogre local0.info discover: onie_disco_eth_addr = 00:04:9F:02:80:A4
Nov 20 02:15:13 ogre local0.info discover: onie_disco_serial_num = fake-serial-0123456789
Nov 20 02:15:13 ogre local0.info discover: onie_disco_url = http://server-04/onie/demo-installer-powerpc-fsl_p2020rdbpca-r0.bin
Nov 20 02:15:13 ogre local0.info discover: onie_disco_lease = 43200
Nov 20 02:15:13 ogre local0.info discover: onie_disco_mask = 24
Nov 20 02:15:13 ogre local0.info discover: onie_disco_hostname = ogre
Nov 20 02:15:13 ogre local0.info discover: onie_disco_wwwsrv = 10.0.1.248
Nov 20 02:15:13 ogre local0.info discover: onie_disco_opt53 = 05

This information is logged using syslogd which is configured to rotate
the log files when they exceed 200KB and keeps a total of 3 log files.

Reviewed By: Vidya Ravipati <vidya@cumulusnetworks.com>
Signed-off-by: Curt Brune <curt@cumulusnetworks.com>