bootstd: Add support for bootflows

Add support for bootflows, including maintaining a list of them and
iterating to find them.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/Makefile b/boot/Makefile
index 1150051..ac861fd 100644
--- a/boot/Makefile
+++ b/boot/Makefile
@@ -20,6 +20,7 @@
 obj-$(CONFIG_ANDROID_BOOT_IMAGE) += image-android.o image-android-dt.o
 
 obj-$(CONFIG_$(SPL_TPL_)BOOTSTD) += bootdev-uclass.o
+obj-$(CONFIG_$(SPL_TPL_)BOOTSTD) += bootflow.o
 obj-$(CONFIG_$(SPL_TPL_)BOOTSTD) += bootmeth-uclass.o
 obj-$(CONFIG_$(SPL_TPL_)BOOTSTD) += bootstd-uclass.o