dhcp: Print IP address by default

Current OSv implementation suppress most of output when it's not verbose mode.

It may speed up bootup speed, but supress printing out IP address is
inconvenient for most of users.  So I added infof(), which acts like debugf()
but print out string even not on verbose mode, and use it in dhcp.cc to print
out IP address.

Signed-off-by: Takuya ASADA <syuu@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>