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/chromebook_link64_defconfig b/configs/chromebook_link64_defconfig
index 2403860..d65b5c6 100644
--- a/configs/chromebook_link64_defconfig
+++ b/configs/chromebook_link64_defconfig
@@ -55,6 +55,7 @@
 CONFIG_CMD_EXT4_WRITE=y
 CONFIG_CMD_FAT=y
 CONFIG_CMD_FS_GENERIC=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_REGMAP=y
 CONFIG_SYSCON=y