env: Allow to build multiple environments in Kconfig

Now that we have everything in place in the code, let's allow to build
multiple environments backend through Kconfig.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
diff --git a/configs/MPC8313ERDB_NAND_66_defconfig b/configs/MPC8313ERDB_NAND_66_defconfig
index 2639926..0f2a675 100644
--- a/configs/MPC8313ERDB_NAND_66_defconfig
+++ b/configs/MPC8313ERDB_NAND_66_defconfig
@@ -22,6 +22,7 @@
 CONFIG_CMD_MTDPARTS=y
 CONFIG_MTDIDS_DEFAULT="nand0=e2800000.flash"
 CONFIG_MTDPARTS_DEFAULT="mtdparts=e2800000.flash:512k(uboot),128k(env),6m@1m(kernel),-(fs)"
+# CONFIG_ENV_IS_IN_FLASH is not set
 CONFIG_ENV_IS_IN_NAND=y
 # CONFIG_MMC is not set
 CONFIG_MTD_NOR_FLASH=y