Convert CONFIG_ETHPRIME to Kconfig

This converts the following to Kconfig:
   CONFIG_ETHPRIME

This is also done by adding a gating Kconfig option, CONFIG_USE_ETHPRIME
similar to other options that are not always set and control environment
variables.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/ids8313_defconfig b/configs/ids8313_defconfig
index ee7faf9..e2785cb 100644
--- a/configs/ids8313_defconfig
+++ b/configs/ids8313_defconfig
@@ -161,6 +161,8 @@
 CONFIG_ENV_ADDR_REDUND=0xFFFE0000
 CONFIG_USE_BOOTFILE=y
 CONFIG_BOOTFILE="ids8313/uImage"
+CONFIG_USE_ETHPRIME=y
+CONFIG_ETHPRIME="TSEC1"
 CONFIG_VERSION_VARIABLE=y
 CONFIG_BOOTCOUNT_LIMIT=y
 CONFIG_BOOTCOUNT_I2C=y