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/topic_miami_defconfig b/configs/topic_miami_defconfig
index dda24d7..9aa8b48 100644
--- a/configs/topic_miami_defconfig
+++ b/configs/topic_miami_defconfig
@@ -34,6 +34,7 @@
 # CONFIG_CMD_NET is not set
 CONFIG_CMD_CACHE=y
 CONFIG_OF_EMBED=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_SPL_DM_SEQ_ALIAS=y
 CONFIG_DFU_RAM=y