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/env/Kconfig b/env/Kconfig
index 443f2f7..2f625b2 100644
--- a/env/Kconfig
+++ b/env/Kconfig
@@ -835,6 +835,18 @@
 	help
 	  The value to set the "bootfile" variable to.
 
+config USE_ETHPRIME
+	bool "Add an 'ethprime' environment variable"
+	help
+	  The "ethprime" variable is used in some cases to control which
+	  network interface is used first.
+
+config ETHPRIME
+	string "'ethprime' environment variable value"
+	depends on USE_ETHPRIME
+	help
+	  The value to set the "ethprime" variable to.
+
 config VERSION_VARIABLE
 	bool "Add a 'ver' environment variable with the U-Boot version"
 	help