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/p2571_defconfig b/configs/p2571_defconfig
index ebf5209..608300a 100644
--- a/configs/p2571_defconfig
+++ b/configs/p2571_defconfig
@@ -23,6 +23,7 @@
 CONFIG_BOOTP_PREFER_SERVERIP=y
 # CONFIG_CMD_NFS is not set
 CONFIG_CMD_EXT4_WRITE=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_DFU_MMC=y
 CONFIG_DFU_RAM=y