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/aristainetos2ccslb_defconfig b/configs/aristainetos2ccslb_defconfig
index d914e4d..9019c82 100644
--- a/configs/aristainetos2ccslb_defconfig
+++ b/configs/aristainetos2ccslb_defconfig
@@ -62,6 +62,8 @@
 CONFIG_ENV_APPEND=y
 CONFIG_ENV_WRITEABLE_LIST=y
 CONFIG_ENV_ACCESS_IGNORE_FORCE=y
+CONFIG_USE_ETHPRIME=y
+CONFIG_ETHPRIME="FEC"
 CONFIG_VERSION_VARIABLE=y
 CONFIG_ARP_TIMEOUT=200
 CONFIG_BOUNCE_BUFFER=y