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/lx2160ardb_tfa_stmm_defconfig b/configs/lx2160ardb_tfa_stmm_defconfig
index ae1ff64..799fc12 100644
--- a/configs/lx2160ardb_tfa_stmm_defconfig
+++ b/configs/lx2160ardb_tfa_stmm_defconfig
@@ -37,6 +37,7 @@
 CONFIG_CMD_EFIDEBUG=y
 CONFIG_MP=y
 CONFIG_OF_CONTROL=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_ENV_ADDR=0x20500000