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/P2020RDB-PC_NAND_defconfig b/configs/P2020RDB-PC_NAND_defconfig
index 9e0de44..6920f4f 100644
--- a/configs/P2020RDB-PC_NAND_defconfig
+++ b/configs/P2020RDB-PC_NAND_defconfig
@@ -58,6 +58,8 @@
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_USE_BOOTFILE=y
 CONFIG_BOOTFILE="uImage"
+CONFIG_USE_ETHPRIME=y
+CONFIG_ETHPRIME="eTSEC1"
 CONFIG_DM=y
 CONFIG_DDR_CLK_FREQ=66666666
 CONFIG_CHIP_SELECTS_PER_CTRL=1