Convert CONFIG_HOSTNAME et al to Kconfig

This converts the following to Kconfig:
   CONFIG_GATEWAYIP
   CONFIG_HOSTNAME
   CONFIG_IPADDR
   CONFIG_NETMASK
   CONFIG_ROOTPATH
   CONFIG_SERVERIP
   CONFIG_UBOOTPATH

To do this, we introduce a CONFIG_USE_ form of each of the above and
change include/env_default.h to test for that to be set before setting a
value. Further, we don't want to stringify the IP address related values
as they are now properly strings via Kconfig.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/M5253DEMO_defconfig b/configs/M5253DEMO_defconfig
index 2684ce0..4de6290 100644
--- a/configs/M5253DEMO_defconfig
+++ b/configs/M5253DEMO_defconfig
@@ -26,6 +26,8 @@
 CONFIG_CMD_FAT=y
 CONFIG_MAC_PARTITION=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_USE_HOSTNAME=y
+CONFIG_HOSTNAME="M5253DEMO"
 # CONFIG_BLOCK_CACHE is not set
 CONFIG_SYS_IDE_MAXBUS=1
 CONFIG_SYS_ATA_STRIDE=4