Convert CONFIG_ENV_IS_IN_SPI_FLASH et al to Kconfig

This converts the following to Kconfig:
   CONFIG_ENV_IS_IN_SPI_FLASH
   CONFIG_ENV_IS_NOWHERE
   CONFIG_ENV_OFFSET
   CONFIG_ENV_OVERWRITE
   CONFIG_ENV_SECT_SIZE
   CONFIG_SYS_MMC_ENV_DEV
   CONFIG_SYS_MMC_ENV_PART

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/imx8mn_beacon_defconfig b/configs/imx8mn_beacon_defconfig
index 3f5e23f..52c7a7e 100644
--- a/configs/imx8mn_beacon_defconfig
+++ b/configs/imx8mn_beacon_defconfig
@@ -61,6 +61,7 @@
 CONFIG_OF_CONTROL=y
 CONFIG_SPL_OF_CONTROL=y
 CONFIG_OF_SPL_REMOVE_PROPS="interrupt-parent interrupts"
+CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_SYS_MMC_ENV_DEV=2