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/ls1021aqds_sdcard_ifc_defconfig b/configs/ls1021aqds_sdcard_ifc_defconfig
index 29045bf..c390b6b 100644
--- a/configs/ls1021aqds_sdcard_ifc_defconfig
+++ b/configs/ls1021aqds_sdcard_ifc_defconfig
@@ -67,6 +67,8 @@
 CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_USE_ETHPRIME=y
+CONFIG_ETHPRIME="eTSEC1"
 CONFIG_DM=y
 CONFIG_SATA=y
 CONFIG_SATA_CEVA=y