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/MPC837XERDB_defconfig b/configs/MPC837XERDB_defconfig
index c21caee..e83097b 100644
--- a/configs/MPC837XERDB_defconfig
+++ b/configs/MPC837XERDB_defconfig
@@ -171,6 +171,8 @@
 CONFIG_ENV_ADDR=0xFE080000
 CONFIG_USE_BOOTFILE=y
 CONFIG_BOOTFILE="uImage"
+CONFIG_USE_ETHPRIME=y
+CONFIG_ETHPRIME="TSEC0"
 CONFIG_DM=y
 CONFIG_FSL_SATA=y
 CONFIG_SYS_SATA_MAX_DEVICE=2