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/net/Kconfig b/net/Kconfig
index e5b5837..2ae9d6a 100644
--- a/net/Kconfig
+++ b/net/Kconfig
@@ -116,4 +116,11 @@
 	  is wrong then the packet is discussed and an error is shown, like
 	  "UDP wrong checksum 29374a23 30ff3826"
 
+config BOOTP_SERVERIP
+	bool "Use the 'serverip' env var for tftp, not bootp"
+	help
+	  Enable this if the TFTP server will be the 'serverip' environment
+	  variable, not the BOOTP server. This affects the operation of both
+	  bootp and tftp.
+
 endif   # if NET