bootstd: Drop some TODOs

The existing TODOs are done, so remove them. Add another that came up
today.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/develop/bootstd.rst b/doc/develop/bootstd.rst
index 6172dc9..51cd573 100644
--- a/doc/develop/bootstd.rst
+++ b/doc/develop/bootstd.rst
@@ -781,9 +781,7 @@
 
 Some things that need to be done to completely replace the distro-boot scripts:
 
-- add bootdev drivers for dhcp, sata, scsi, ide, virtio
-- PXE boot for EFI
-- support for loading U-Boot scripts
+- implement extensions (devicetree overlays with add-on boards)
 
 Other ideas: