net: move CONFIG_NET guard to top Makefile

When CONFIG_NET is disabled, nothing under the net/ directory is
compiled. Move the CONFIG_NET guard to the top Makefile so the net/
directory is entirely skipped.

When Kbuild visits net/Makefile, CONFIG_NET is obvioulsy 'y' because
CONFIG_NET is a bool option. Clean up net/Makefile.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20210125231659.106201-1-masahiroy@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

Makefile

@@ -649,7 +649,8 @@ ifeq ($(KBUILD_EXTMOD),)
 core-y		:= init/ usr/
 drivers-y	:= drivers/ sound/
 drivers-$(CONFIG_SAMPLES) += samples/
-drivers-y	+= net/ virt/
+drivers-$(CONFIG_NET) += net/
+drivers-y	+= virt/
 libs-y		:= lib/
 endif # KBUILD_EXTMOD
 

net/Makefile

@@ -6,20 +6,19 @@
 # Rewritten to use lists instead of if-statements.
 #
 
-obj-$(CONFIG_NET)		:= devres.o socket.o core/
+obj-y				:= devres.o socket.o core/
 
-tmp-$(CONFIG_COMPAT) 		:= compat.o
-obj-$(CONFIG_NET)		+= $(tmp-y)
+obj-$(CONFIG_COMPAT)		+= compat.o
 
 # LLC has to be linked before the files in net/802/
 obj-$(CONFIG_LLC)		+= llc/
-obj-$(CONFIG_NET)		+= ethernet/ 802/ sched/ netlink/ bpf/ ethtool/
+obj-y				+= ethernet/ 802/ sched/ netlink/ bpf/ ethtool/
 obj-$(CONFIG_NETFILTER)		+= netfilter/
 obj-$(CONFIG_INET)		+= ipv4/
 obj-$(CONFIG_TLS)		+= tls/
 obj-$(CONFIG_XFRM)		+= xfrm/
 obj-$(CONFIG_UNIX_SCM)		+= unix/
-obj-$(CONFIG_NET)		+= ipv6/
+obj-y				+= ipv6/
 obj-$(CONFIG_BPFILTER)		+= bpfilter/
 obj-$(CONFIG_PACKET)		+= packet/
 obj-$(CONFIG_NET_KEY)		+= key/
@@ -63,9 +62,7 @@ obj-$(CONFIG_6LOWPAN)		+= 6lowpan/
 obj-$(CONFIG_IEEE802154)	+= ieee802154/
 obj-$(CONFIG_MAC802154)		+= mac802154/
 
-ifeq ($(CONFIG_NET),y)
 obj-$(CONFIG_SYSCTL)		+= sysctl_net.o
-endif
 obj-$(CONFIG_DNS_RESOLVER)	+= dns_resolver/
 obj-$(CONFIG_CEPH_LIB)		+= ceph/
 obj-$(CONFIG_BATMAN_ADV)	+= batman-adv/