[slaac] add config option to enable/disable SLAAC support (#3512)

This commit adds `OPENTHREAD_CONFIG_ENABLE_SLAAC` as a build
configuration option to enable/disable SLAAC support within
OpenThread. This feature is disabled by default. This commit also
adds `SLAAC` to `common-switches` list to allow builds to enable
this feature easily.

examples/common-switches.mk

@@ -26,6 +26,39 @@
 #  POSSIBILITY OF SUCH DAMAGE.
 #
 
+# OpenThread Features (Makefile default configuration).
+
+BORDER_AGENT        ?= 0
+BORDER_ROUTER       ?= 0
+CERT_LOG            ?= 0
+COAP                ?= 0
+COAPS               ?= 0
+COMMISSIONER        ?= 0
+COVERAGE            ?= 0
+CHANNEL_MANAGER     ?= 0
+CHANNEL_MONITOR     ?= 0
+CHILD_SUPERVISION   ?= 0
+DEBUG               ?= 0
+DHCP6_CLIENT        ?= 0
+DHCP6_SERVER        ?= 0
+DIAGNOSTIC          ?= 0
+DISABLE_DOC         ?= 0
+DNS_CLIENT          ?= 0
+ECDSA               ?= 0
+JAM_DETECTION       ?= 0
+JOINER              ?= 0
+LEGACY              ?= 0
+LINK_RAW            ?= 0
+MAC_FILTER          ?= 0
+MTD_NETDIAG         ?= 0
+PLATFORM_UDP        ?= 0
+SERVICE             ?= 0
+SLAAC               ?= 1  # SLAAC is enabled by default
+SNTP_CLIENT         ?= 0
+TIME_SYNC           ?= 0
+UDP_FORWARD         ?= 0
+
+
 ifeq ($(BORDER_AGENT),1)
 configure_OPTIONS              += --enable-border-agent
 endif
@@ -127,6 +160,10 @@ ifeq ($(SERVICE),1)
 configure_OPTIONS              += --enable-service
 endif
 
+ifeq ($(SLAAC),1)
+COMMONCFLAGS                   += -DOPENTHREAD_CONFIG_ENABLE_SLAAC=1
+endif
+
 ifeq ($(SNTP_CLIENT),1)
 configure_OPTIONS              += --enable-sntp-client
 endif

examples/platforms/posix/openthread-core-posix-config.h

@@ -53,6 +53,16 @@
 #define OPENTHREAD_CONFIG_LOG_OUTPUT OPENTHREAD_CONFIG_LOG_OUTPUT_PLATFORM_DEFINED
 #endif
 
+/**
+ * @def OPENTHREAD_CONFIG_ENABLE_SLAAC
+ *
+ * Define as 1 to enable support for adding of auto-configured SLAAC addresses by OpenThread.
+ *
+ */
+#ifndef OPENTHREAD_CONFIG_ENABLE_SLAAC /* allows command line override */
+#define OPENTHREAD_CONFIG_ENABLE_SLAAC 1
+#endif
+
 #if OPENTHREAD_RADIO
 /**
  * @def OPENTHREAD_CONFIG_ENABLE_SOFTWARE_ACK_TIMEOUT