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/Cyrus_P5020_defconfig b/configs/Cyrus_P5020_defconfig
index d7798ef..a488ad8 100644
--- a/configs/Cyrus_P5020_defconfig
+++ b/configs/Cyrus_P5020_defconfig
@@ -26,6 +26,7 @@
 # CONFIG_CMD_HASH is not set
 CONFIG_CMD_EXT2=y
 CONFIG_CMD_FAT=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_FSL_ESDHC=y