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/socfpga_mcvevk_defconfig b/configs/socfpga_mcvevk_defconfig
index 312dbc9..fced45b 100644
--- a/configs/socfpga_mcvevk_defconfig
+++ b/configs/socfpga_mcvevk_defconfig
@@ -33,6 +33,7 @@
 CONFIG_CMD_EXT4_WRITE=y
 # CONFIG_ISO_PARTITION is not set
 # CONFIG_EFI_PARTITION is not set
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_DFU_MMC=y