Convert CONFIG_KEEP_SERVERADDR to Kconfig

This converts the following to Kconfig:
   CONFIG_KEEP_SERVERADDR

Drop the preprocessor usage also.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/net/Kconfig b/net/Kconfig
index cabe93c..8e9579a 100644
--- a/net/Kconfig
+++ b/net/Kconfig
@@ -99,4 +99,13 @@
 		receiving response from main DHCP server. Has no effect if
 		SERVERIP_FROM_PROXYDHCP is false.
 
+config KEEP_SERVERADDR
+	bool "Write the server's MAC address to 'serveraddr'"
+	default y if SANDBOX
+	help
+	  Keeps the server's MAC address, in the env 'serveraddr'
+	  for passing to bootargs (like Linux's netconsole option). If this is
+	  enabled, when an ARP reply is received, the server's IP address is
+	  written there.
+
 endif   # if NET