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/stm32mp15_trusted_defconfig b/configs/stm32mp15_trusted_defconfig
index 3309c2e..e00c00f 100644
--- a/configs/stm32mp15_trusted_defconfig
+++ b/configs/stm32mp15_trusted_defconfig
@@ -67,6 +67,8 @@
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_SYS_MMC_ENV_DEV=-1
 CONFIG_TFTP_TSIZE=y
+CONFIG_USE_SERVERIP=y
+CONFIG_SERVERIP="192.168.1.1"
 CONFIG_STM32_ADC=y
 CONFIG_CLK_SCMI=y
 CONFIG_SET_DFU_ALT_INFO=y