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/puma-rk3399_defconfig b/configs/puma-rk3399_defconfig
index 987f842..0fed34d 100644
--- a/configs/puma-rk3399_defconfig
+++ b/configs/puma-rk3399_defconfig
@@ -40,6 +40,7 @@
 CONFIG_SPL_OF_CONTROL=y
 CONFIG_OF_LIVE=y
 CONFIG_OF_SPL_REMOVE_PROPS="pinctrl-0 pinctrl-names interrupt-parent assigned-clocks assigned-clock-rates assigned-clock-parents"
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_ROCKCHIP_GPIO=y