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/tqma6dl_mba6_spi_defconfig b/configs/tqma6dl_mba6_spi_defconfig
index c9f17a8..25f231c 100644
--- a/configs/tqma6dl_mba6_spi_defconfig
+++ b/configs/tqma6dl_mba6_spi_defconfig
@@ -40,6 +40,8 @@
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_SYS_REDUNDAND_ENVIRONMENT=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_USE_ETHPRIME=y
+CONFIG_ETHPRIME="FEC"
 CONFIG_ARP_TIMEOUT=200
 CONFIG_BOUNCE_BUFFER=y
 CONFIG_I2C_SET_DEFAULT_BUS_NUM=y