Convert CONFIG_BOOTP_SERVERIP to Kconfig

This converts the following to Kconfig:
   CONFIG_BOOTP_SERVERIP

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/adp-ae3xx_defconfig b/configs/adp-ae3xx_defconfig
index b79b5b6f..9f7b268 100644
--- a/configs/adp-ae3xx_defconfig
+++ b/configs/adp-ae3xx_defconfig
@@ -31,6 +31,7 @@
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_BOOTP_SEND_HOSTNAME=y
 CONFIG_NET_RANDOM_ETHADDR=y
+CONFIG_BOOTP_SERVERIP=y
 CONFIG_DM=y
 CONFIG_CLK=y
 CONFIG_MMC=y