Convert CONFIG_ENV_OVERWRITE to Kconfig

This converts the following to Kconfig:
   CONFIG_ENV_OVERWRITE

Signed-off-by: Adam Ford <aford173@gmail.com>
[trini: Rerun migration, remove some comments]
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/nokia_rx51_defconfig b/configs/nokia_rx51_defconfig
index c2f12c9..653262e 100644
--- a/configs/nokia_rx51_defconfig
+++ b/configs/nokia_rx51_defconfig
@@ -39,6 +39,7 @@
 CONFIG_CMD_FAT=y
 CONFIG_MTDIDS_DEFAULT="onenand0=onenand"
 CONFIG_MTDPARTS_DEFAULT="mtdparts=onenand:128k(bootloader)ro,384k(config),256k(log),2m(kernel),2m(initfs),-(rootfs)"
+CONFIG_ENV_OVERWRITE=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 # CONFIG_NET is not set
 CONFIG_TWL4030_LED=y