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/lx2160aqds_tfa_defconfig b/configs/lx2160aqds_tfa_defconfig
index f12ad8f..a9887ad 100644
--- a/configs/lx2160aqds_tfa_defconfig
+++ b/configs/lx2160aqds_tfa_defconfig
@@ -54,6 +54,8 @@
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_ENV_ADDR=0x20500000
+CONFIG_USE_ETHPRIME=y
+CONFIG_ETHPRIME="DPMAC17@rgmii-id"
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_DM=y
 CONFIG_SATA=y