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/mt8512_bm1_emmc_defconfig b/configs/mt8512_bm1_emmc_defconfig
index 487076b..10a2083 100644
--- a/configs/mt8512_bm1_emmc_defconfig
+++ b/configs/mt8512_bm1_emmc_defconfig
@@ -14,6 +14,7 @@
 CONFIG_SYS_PROMPT="MT8512> "
 CONFIG_CMD_BOOTMENU=y
 CONFIG_CMD_MMC=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_REGMAP=y
 CONFIG_SYSCON=y
 CONFIG_CLK=y