build: add CONFIG_YAML

Makefile

@@ -81,6 +81,15 @@ MULTICALL_${CONFIG_IFPARSE}_OBJ += ${IFPARSE_SRC:.c=.o}
 CMDS_${CONFIG_IFPARSE} += ifparse
 CPPFLAGS_${CONFIG_IFPARSE} += -DCONFIG_IFPARSE
 
+# enable YAML support (+? KB)
+CONFIG_YAML ?= Y
+YAML_SRC = \
+	libifupdown/yaml-base.c \
+	libifupdown/yaml-writer.c
+LIBIFUPDOWN_${CONFIG_YAML}_OBJ += ${YAML_SRC:.c=.o}
+CPPFLAGS_${CONFIG_YAML} += -DCONFIG_YAML
+
+LIBIFUPDOWN_OBJ += ${LIBIFUPDOWN_Y_OBJ}
 MULTICALL_OBJ += ${MULTICALL_Y_OBJ}
 CMDS += ${CMDS_Y}
 CPPFLAGS += ${CPPFLAGS_Y}