bootstd: sandbox: Add a hostfs bootdev

It is helpful to be able to try out bootstd on sandbox, using host files.
This is easier than using a block device, which must have a filesystem,
partition table, etc.

Add a new driver which provides this feature. For now it is not used in
tests, but it is likely to be useful.

Add notes in the devicetree also, but don't disturb the tests.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/fs/sandbox/Makefile b/fs/sandbox/Makefile
index 0609051..880d59d 100644
--- a/fs/sandbox/Makefile
+++ b/fs/sandbox/Makefile
@@ -9,3 +9,4 @@
 # Pavel Bartusek, Sysgo Real-Time Solutions AG, pba@sysgo.de
 
 obj-y := sandboxfs.o
+obj-$(CONFIG_$(SPL_TPL_)BOOTSTD) += host_bootdev.o