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/kp_imx6q_tpc_defconfig b/configs/kp_imx6q_tpc_defconfig
index d49b16f..676c5c3 100644
--- a/configs/kp_imx6q_tpc_defconfig
+++ b/configs/kp_imx6q_tpc_defconfig
@@ -45,6 +45,7 @@
 CONFIG_OF_CONTROL=y
 CONFIG_SPL_OF_CONTROL=y
 CONFIG_OF_SPL_REMOVE_PROPS="interrupt-parent assigned-clocks assigned-clock-rates assigned-clock-parents interrupts dmas dma-names"
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_SYS_REDUNDAND_ENVIRONMENT=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y