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/imx8mq_cm_defconfig b/configs/imx8mq_cm_defconfig
index b272070..04edd54 100644
--- a/configs/imx8mq_cm_defconfig
+++ b/configs/imx8mq_cm_defconfig
@@ -47,6 +47,8 @@
 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="FEC"
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_SAVED_DRAM_TIMING_BASE=0x40000000
 CONFIG_MXC_GPIO=y