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/ls1046ardb_emmc_defconfig b/configs/ls1046ardb_emmc_defconfig
index a319bf3..dd3b403 100644
--- a/configs/ls1046ardb_emmc_defconfig
+++ b/configs/ls1046ardb_emmc_defconfig
@@ -59,6 +59,8 @@
 CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_USE_ETHPRIME=y
+CONFIG_ETHPRIME="FM1@DTSEC3"
 CONFIG_DM=y
 CONFIG_SATA=y
 CONFIG_SATA_CEVA=y