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/colibri-imx8x_defconfig b/configs/colibri-imx8x_defconfig
index 6896236..e4c4189 100644
--- a/configs/colibri-imx8x_defconfig
+++ b/configs/colibri-imx8x_defconfig
@@ -47,6 +47,12 @@
 CONFIG_IP_DEFRAG=y
 CONFIG_TFTP_BLOCKSIZE=4096
 CONFIG_TFTP_TSIZE=y
+CONFIG_USE_IPADDR=y
+CONFIG_IPADDR="192.168.10.2"
+CONFIG_USE_NETMASK=y
+CONFIG_NETMASK="255.255.255.0"
+CONFIG_USE_SERVERIP=y
+CONFIG_SERVERIP="192.168.10.1"
 CONFIG_CLK_IMX8=y
 CONFIG_CPU=y
 CONFIG_FXL6408_GPIO=y