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/theadorable_debug_defconfig b/configs/theadorable_debug_defconfig
index 6f4e05a..16efc6e 100644
--- a/configs/theadorable_debug_defconfig
+++ b/configs/theadorable_debug_defconfig
@@ -49,6 +49,7 @@
 CONFIG_EFI_PARTITION=y
 # CONFIG_PARTITION_UUIDS is not set
 # CONFIG_SPL_PARTITION_UUIDS is not set
+CONFIG_ENV_OVERWRITE=y
 CONFIG_USE_ENV_SPI_MAX_HZ=y
 CONFIG_ENV_SPI_MAX_HZ=50000000
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y