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/mx6memcal_defconfig b/configs/mx6memcal_defconfig
index 15e10ea..5e7d0ea 100644
--- a/configs/mx6memcal_defconfig
+++ b/configs/mx6memcal_defconfig
@@ -37,6 +37,7 @@
 # CONFIG_CMD_LOADB is not set
 # CONFIG_CMD_LOADS is not set
 CONFIG_CMD_CACHE=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 # CONFIG_NET is not set
 CONFIG_DM=y