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/socfpga_vining_fpga_defconfig b/configs/socfpga_vining_fpga_defconfig
index cdad36b..6d859b3 100644
--- a/configs/socfpga_vining_fpga_defconfig
+++ b/configs/socfpga_vining_fpga_defconfig
@@ -63,6 +63,8 @@
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_USE_BOOTFILE=y
 CONFIG_BOOTFILE="fitImage"
+CONFIG_USE_HOSTNAME=y
+CONFIG_HOSTNAME="socfpga_vining_fpga"
 CONFIG_VERSION_VARIABLE=y
 CONFIG_BOOTP_SEND_HOSTNAME=y
 CONFIG_NET_RANDOM_ETHADDR=y