Convert CONFIG_SPL_ENV_SUPPORT to Kconfig

Move this option to Kconfig and tidy up existing uses.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/B4420QDS_NAND_defconfig b/configs/B4420QDS_NAND_defconfig
index 5a88ada..c0bcf16 100644
--- a/configs/B4420QDS_NAND_defconfig
+++ b/configs/B4420QDS_NAND_defconfig
@@ -1,4 +1,5 @@
 CONFIG_PPC=y
+CONFIG_SPL_ENV_SUPPORT=y
 CONFIG_MPC85xx=y
 CONFIG_TARGET_B4860QDS=y
 CONFIG_FIT=y