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/cm_fx6_defconfig b/configs/cm_fx6_defconfig
index 8ae915d..cfa3281 100644
--- a/configs/cm_fx6_defconfig
+++ b/configs/cm_fx6_defconfig
@@ -65,6 +65,8 @@
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y
+CONFIG_USE_ETHPRIME=y
+CONFIG_ETHPRIME="FEC0"
 CONFIG_ARP_TIMEOUT=200
 CONFIG_SPL_DM=y
 CONFIG_BOUNCE_BUFFER=y