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_tfa_SECURE_BOOT_defconfig b/configs/ls1043ardb_tfa_SECURE_BOOT_defconfig
index 4b0ef66..41a285e 100644
--- a/configs/ls1043ardb_tfa_SECURE_BOOT_defconfig
+++ b/configs/ls1043ardb_tfa_SECURE_BOOT_defconfig
@@ -36,6 +36,8 @@
 CONFIG_MTDPARTS_DEFAULT="mtdparts=60000000.nor:2m@0x100000(nor_bank0_uboot),40m@0x1100000(nor_bank0_fit),7m(nor_bank0_user),2m@0x4100000(nor_bank4_uboot),40m@0x5100000(nor_bank4_fit),-(nor_bank4_user);7e800000.flash:1m(nand_uboot),1m(nand_uboot_env),20m(nand_fit);spi0.0:1m(uboot),5m(kernel),1m(dtb),9m(file_system)"
 CONFIG_OF_CONTROL=y
 CONFIG_ENV_OVERWRITE=y
+CONFIG_USE_ETHPRIME=y
+CONFIG_ETHPRIME="FM1@DTSEC3"
 CONFIG_DM=y
 # CONFIG_DDR_SPD is not set
 CONFIG_ECC_INIT_VIA_DDRCONTROLLER=y