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/flea3_defconfig b/configs/flea3_defconfig
index c82fef1..30c0b2e 100644
--- a/configs/flea3_defconfig
+++ b/configs/flea3_defconfig
@@ -23,6 +23,7 @@
 CONFIG_CMD_MTDPARTS=y
 CONFIG_MTDIDS_DEFAULT="nand0=mxc_nand,nor0=physmap-flash.0"
 CONFIG_MTDPARTS_DEFAULT="mtdparts=mxc_nand:50m(root1),32m(rootfb),64m(pcache),64m(app1),10m(app2),-(spool);physmap-flash.0:512k(u-boot),64k(env1),64k(env2),3776k(kernel1),3776k(kernel2)"
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_SYS_REDUNDAND_ENVIRONMENT=y
 CONFIG_ENV_ADDR=0xA0080000