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/board/ti/am335x/Kconfig b/board/ti/am335x/Kconfig
index 97374bd..0a0cbf6 100644
--- a/board/ti/am335x/Kconfig
+++ b/board/ti/am335x/Kconfig
@@ -1,5 +1,8 @@
 if TARGET_AM335X_EVM
 
+config SPL_ENV_SUPPORT
+	default y
+
 config SYS_BOARD
 	default "am335x"