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/ls2081ardb_defconfig b/configs/ls2081ardb_defconfig
index a2a1419..7173aaa 100644
--- a/configs/ls2081ardb_defconfig
+++ b/configs/ls2081ardb_defconfig
@@ -42,6 +42,8 @@
 CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_USE_ETHPRIME=y
+CONFIG_ETHPRIME="DPMAC1@xgmii"
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_DM=y
 CONFIG_SATA=y