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/sniper_defconfig b/configs/sniper_defconfig
index f11ea86..ebfb2a5 100644
--- a/configs/sniper_defconfig
+++ b/configs/sniper_defconfig
@@ -20,6 +20,7 @@
 CONFIG_CMD_MMC=y
 # CONFIG_CMD_SETEXPR is not set
 # CONFIG_CMD_NFS is not set
+CONFIG_ENV_OVERWRITE=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_USB_FUNCTION_FASTBOOT=y
 CONFIG_FASTBOOT_BUF_ADDR=0x82000000