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/sbc8548_PCI_66_PCIE_defconfig b/configs/sbc8548_PCI_66_PCIE_defconfig
index 97474a2..5f8b292 100644
--- a/configs/sbc8548_PCI_66_PCIE_defconfig
+++ b/configs/sbc8548_PCI_66_PCIE_defconfig
@@ -21,6 +21,7 @@
 CONFIG_CMD_PING=y
 # CONFIG_CMD_HASH is not set
 # CONFIG_CMD_IRQ is not set
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_ADDR=0xFFFE0000
 # CONFIG_MMC is not set
 CONFIG_MTD_NOR_FLASH=y