Convert CONFIG_ENV_RANGE to Kconfig

This converts the following to Kconfig:
   CONFIG_ENV_RANGE

Now that this is in Kconfig we can enforce a minimum size and so remove
the check in C code to ensure range is larger than size.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/thuban_defconfig b/configs/thuban_defconfig
index 0a6b0e8..6115e9e 100644
--- a/configs/thuban_defconfig
+++ b/configs/thuban_defconfig
@@ -79,6 +79,7 @@
 CONFIG_OF_EMBED=y
 CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_NAND=y
+CONFIG_ENV_RANGE=0x80000
 CONFIG_SYS_REDUNDAND_ENVIRONMENT=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_NET_RETRY_COUNT=10