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/P2020RDB-PC_NAND_defconfig b/configs/P2020RDB-PC_NAND_defconfig
index e6cd51c..5a62055 100644
--- a/configs/P2020RDB-PC_NAND_defconfig
+++ b/configs/P2020RDB-PC_NAND_defconfig
@@ -71,6 +71,7 @@
 CONFIG_OF_CONTROL=y
 CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_NAND=y
+CONFIG_ENV_RANGE=0xc000
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_USE_BOOTFILE=y
 CONFIG_BOOTFILE="uImage"