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/stmark2_defconfig b/configs/stmark2_defconfig
index 5ee2edc..ae7a9cf 100644
--- a/configs/stmark2_defconfig
+++ b/configs/stmark2_defconfig
@@ -37,6 +37,8 @@
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_ENV_SPI_CS=1
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_USE_HOSTNAME=y
+CONFIG_HOSTNAME="stmark2"
 # CONFIG_NET is not set
 CONFIG_MTD=y
 CONFIG_DM_SPI_FLASH=y