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/cherryhill_defconfig b/configs/cherryhill_defconfig
index 321121e..83d5712 100644
--- a/configs/cherryhill_defconfig
+++ b/configs/cherryhill_defconfig
@@ -43,6 +43,7 @@
 CONFIG_USE_BOOTFILE=y
 CONFIG_BOOTFILE="bzImage"
 CONFIG_TFTP_TSIZE=y
+CONFIG_USE_ROOTPATH=y
 CONFIG_REGMAP=y
 CONFIG_SYSCON=y
 # CONFIG_ACPIGEN is not set