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/ls1043ardb_defconfig b/configs/ls1043ardb_defconfig
index 173109e..3fa35c5 100644
--- a/configs/ls1043ardb_defconfig
+++ b/configs/ls1043ardb_defconfig
@@ -37,6 +37,8 @@
 CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_ENV_ADDR=0x60300000
+CONFIG_USE_ETHPRIME=y
+CONFIG_ETHPRIME="FM1@DTSEC3"
 CONFIG_DM=y
 CONFIG_FSL_CAAM=y
 # CONFIG_DDR_SPD is not set