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/pg_wcom_seli8_update_defconfig b/configs/pg_wcom_seli8_update_defconfig
index 1ca468a..f039b78 100644
--- a/configs/pg_wcom_seli8_update_defconfig
+++ b/configs/pg_wcom_seli8_update_defconfig
@@ -57,6 +57,8 @@
 CONFIG_SYS_REDUNDAND_ENVIRONMENT=y
 CONFIG_ENV_ADDR=0x60220000
 CONFIG_ENV_ADDR_REDUND=0x60200000
+CONFIG_USE_ETHPRIME=y
+CONFIG_ETHPRIME="ethernet@2d90000"
 CONFIG_VERSION_VARIABLE=y
 CONFIG_DM=y
 CONFIG_BOOTCOUNT_LIMIT=y