netlink: add NETLINK to the DEFAULTS for each architecture

NETLINK is going to replace rtsock and a number of other ioctl/sysctl interfaces.
In-base utilies such as route(8), netstat(8) and soon ifconfig(8)
 are being converted to use netlink sockets as a transport between
 kernel and userland.
In the current configuration, it still possible have the kernel
 without NETLINK (`nooptions NETLINK`) and use the aforementioned
 utilies by buidling the world with `WITHOUT_NETLINK` src.conf knob.
However, this approach does not cover the cases when person unintentionally
 builds a custom kernel without netlink and tries to use the standard userland.

This change adds `option NETLINK` to the default options for each
 architecture, fixing the custom kernel issue.
For arm, this change uses `std.armv6` and `std.armv7` (netlink already in)
 instead of DEFAULTS.

Reviewed By: imp
Differential Revision: https://reviews.freebsd.org/D39339

sys/arm/conf/std.armv6

@@ -11,6 +11,7 @@ options 	CC_CUBIC		# include CUBIC congestion control
 options 	TCP_HHOOK		# hhook(9) framework for TCP
 device		crypto			# core crypto support
 options 	IPSEC_SUPPORT		# Allow kldload of ipsec and tcpmd5
+options 	NETLINK			# netlink(4) support
 options 	SCTP_SUPPORT		# Allow kldload of SCTP
 options 	FFS			# Berkeley Fast Filesystem
 options 	SOFTUPDATES		# Enable FFS soft updates support