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/MPC8548CDS_legacy_defconfig b/configs/MPC8548CDS_legacy_defconfig
index ae262bf..123dd20 100644
--- a/configs/MPC8548CDS_legacy_defconfig
+++ b/configs/MPC8548CDS_legacy_defconfig
@@ -31,6 +31,8 @@
 CONFIG_ENV_ADDR=0xFFF60000
 CONFIG_USE_BOOTFILE=y
 CONFIG_BOOTFILE="8548cds/uImage.uboot"
+CONFIG_USE_ETHPRIME=y
+CONFIG_ETHPRIME="eTSEC0"
 CONFIG_DM=y
 CONFIG_CHIP_SELECTS_PER_CTRL=2
 CONFIG_DDR_ECC=y