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/qemu_mips_defconfig b/configs/qemu_mips_defconfig
index f12fddc..ebf7af0 100644
--- a/configs/qemu_mips_defconfig
+++ b/configs/qemu_mips_defconfig
@@ -16,6 +16,7 @@
 CONFIG_CMD_EXT2=y
 CONFIG_CMD_FAT=y
 # CONFIG_ISO_PARTITION is not set
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_ENV_ADDR=0xBFFF8000
 CONFIG_MTD_NOR_FLASH=y