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/apalis-imx8x_defconfig b/configs/apalis-imx8x_defconfig
index 25da027..a7ce626 100644
--- a/configs/apalis-imx8x_defconfig
+++ b/configs/apalis-imx8x_defconfig
@@ -39,6 +39,8 @@
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_SYS_MMC_ENV_PART=1
 CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y
+CONFIG_USE_ETHPRIME=y
+CONFIG_ETHPRIME="eth0"
 CONFIG_VERSION_VARIABLE=y
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_IP_DEFRAG=y