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/zynq_cse_nor_defconfig b/configs/zynq_cse_nor_defconfig
index 2d84fa6..b748eeb 100644
--- a/configs/zynq_cse_nor_defconfig
+++ b/configs/zynq_cse_nor_defconfig
@@ -46,6 +46,7 @@
 # CONFIG_CMD_SETEXPR is not set
 # CONFIG_CMD_MISC is not set
 CONFIG_OF_EMBED=y
+CONFIG_ENV_OVERWRITE=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 # CONFIG_NET is not set
 # CONFIG_DM_WARN is not set