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/bcm958305k_defconfig b/configs/bcm958305k_defconfig
index 33015dc..5aae4d1 100644
--- a/configs/bcm958305k_defconfig
+++ b/configs/bcm958305k_defconfig
@@ -19,6 +19,7 @@
 CONFIG_CMD_CACHE=y
 CONFIG_CMD_TIME=y
 CONFIG_CMD_FAT=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 # CONFIG_MMC is not set
 CONFIG_PHY_BROADCOM=y