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/mx6ull_14x14_evk_defconfig b/configs/mx6ull_14x14_evk_defconfig
index a053d2f..d587244 100644
--- a/configs/mx6ull_14x14_evk_defconfig
+++ b/configs/mx6ull_14x14_evk_defconfig
@@ -35,6 +35,8 @@
 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="eth1"
 CONFIG_BOUNCE_BUFFER=y
 CONFIG_DM_74X164=y
 CONFIG_DM_I2C=y