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/xilinx_zynqmp_r5_defconfig b/configs/xilinx_zynqmp_r5_defconfig
index 74dc56a..b4e20eb 100644
--- a/configs/xilinx_zynqmp_r5_defconfig
+++ b/configs/xilinx_zynqmp_r5_defconfig
@@ -13,6 +13,7 @@
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_CMD_BOOTSTAGE=y
 CONFIG_OF_EMBED=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_ZYNQ_SERIAL=y
 CONFIG_TIMER=y