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/P1020MBG-PC_36BIT_defconfig b/configs/P1020MBG-PC_36BIT_defconfig
index 2385da4..39abe1f 100644
--- a/configs/P1020MBG-PC_36BIT_defconfig
+++ b/configs/P1020MBG-PC_36BIT_defconfig
@@ -27,6 +27,7 @@
 # CONFIG_CMD_HASH is not set
 CONFIG_CMD_EXT2=y
 CONFIG_CMD_FAT=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_ENV_ADDR=0xEFF20000
 CONFIG_FSL_ESDHC=y