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_evk_defconfig b/configs/imx8mp_evk_defconfig
index 0ff549f..9a8bd02 100644
--- a/configs/imx8mp_evk_defconfig
+++ b/configs/imx8mp_evk_defconfig
@@ -58,6 +58,8 @@
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_SYS_MMC_ENV_DEV=1
 CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y
+CONFIG_USE_ETHPRIME=y
+CONFIG_ETHPRIME="eth1"
 CONFIG_SPL_DM=y
 CONFIG_CLK_COMPOSITE_CCF=y
 CONFIG_CLK_IMX8MP=y