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/kmtepr2_defconfig b/configs/kmtepr2_defconfig
index fff5b1d..48c1a72 100644
--- a/configs/kmtepr2_defconfig
+++ b/configs/kmtepr2_defconfig
@@ -171,6 +171,8 @@
 CONFIG_SYS_REDUNDAND_ENVIRONMENT=y
 CONFIG_ENV_ADDR=0xF00C0000
 CONFIG_ENV_ADDR_REDUND=0xF00E0000
+CONFIG_USE_ETHPRIME=y
+CONFIG_ETHPRIME="UEC0"
 CONFIG_VERSION_VARIABLE=y
 CONFIG_BOOTCOUNT_LIMIT=y
 CONFIG_DM_BOOTCOUNT=y