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/socrates_defconfig b/configs/socrates_defconfig
index e24be7a..1eb72d3 100644
--- a/configs/socrates_defconfig
+++ b/configs/socrates_defconfig
@@ -45,6 +45,8 @@
 CONFIG_SYS_REDUNDAND_ENVIRONMENT=y
 CONFIG_ENV_ADDR=0xFFF40000
 CONFIG_ENV_ADDR_REDUND=0xFFF20000
+CONFIG_USE_ETHPRIME=y
+CONFIG_ETHPRIME="TSEC0"
 CONFIG_DM=y
 CONFIG_CHIP_SELECTS_PER_CTRL=2
 CONFIG_SYS_BR0_PRELIM_BOOL=y