support swap partition

activate the swap partition:

mkswap /dev/mmcblk0p3
swapon /dev/mmcblk0p3

run command 'free -h' to check if the swap partition has been activated.

build/boards/cv180x/cv1800b_milkv_duo_sd/linux/cvitek_cv1800b_milkv_duo_sd_defconfig

-# CONFIG_SWAP is not set
+CONFIG_SWAP=y
 CONFIG_SYSVIPC=y
 CONFIG_POSIX_MQUEUE=y
 CONFIG_NO_HZ_IDLE=y

milkv/genimage-milkv-duo.cfg

@@ -30,4 +30,9 @@ image milkv-duo.img {
 		partition-type = 0x83
 		image = "rootfs.ext4"
 	}
+
+	partition empty {
+		partition-type-uuid = 0657fd6d-a4ab-43c4-84e5-0933c84b4f4f
+		size = 256M
+	}
 }