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/P5040DS_NAND_defconfig b/configs/P5040DS_NAND_defconfig
index 7cd2ae6..0e04aca 100644
--- a/configs/P5040DS_NAND_defconfig
+++ b/configs/P5040DS_NAND_defconfig
@@ -40,6 +40,8 @@
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_USE_BOOTFILE=y
 CONFIG_BOOTFILE="uImage"
+CONFIG_USE_ETHPRIME=y
+CONFIG_ETHPRIME="FM1@DTSEC1"
 CONFIG_DM=y
 CONFIG_SYS_SATA_MAX_DEVICE=2
 CONFIG_FSL_CAAM=y