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/imx8mp_rsb3720a1_4G_defconfig b/configs/imx8mp_rsb3720a1_4G_defconfig
index 04d3634..8af41d5 100644
--- a/configs/imx8mp_rsb3720a1_4G_defconfig
+++ b/configs/imx8mp_rsb3720a1_4G_defconfig
@@ -79,6 +79,8 @@
 CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_SECTOR=0x300
 CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_USE_SECTOR=y
 CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y
+CONFIG_USE_ETHPRIME=y
+CONFIG_ETHPRIME="eth1"
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_SPL_DM=y
 CONFIG_REGMAP=y