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/sandbox_flattree_defconfig b/configs/sandbox_flattree_defconfig
index 4d5a73f..3f69fb7 100644
--- a/configs/sandbox_flattree_defconfig
+++ b/configs/sandbox_flattree_defconfig
@@ -76,6 +76,7 @@
 CONFIG_BOOTP_SEND_HOSTNAME=y
 CONFIG_NETCONSOLE=y
 CONFIG_IP_DEFRAG=y
+CONFIG_BOOTP_SERVERIP=y
 CONFIG_DM_DMA=y
 CONFIG_REGMAP=y
 CONFIG_SYSCON=y