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/P1021RDB-PC_36BIT_SDCARD_defconfig b/configs/P1021RDB-PC_36BIT_SDCARD_defconfig
index 4c86983..1e8f85e 100644
--- a/configs/P1021RDB-PC_36BIT_SDCARD_defconfig
+++ b/configs/P1021RDB-PC_36BIT_SDCARD_defconfig
@@ -1,4 +1,5 @@
 CONFIG_PPC=y
+CONFIG_SPL_ENV_SUPPORT=y
 CONFIG_MPC85xx=y
 CONFIG_TARGET_P1_P2_RDB_PC=y
 CONFIG_PHYS_64BIT=y