Documentation/Makefile.sphinx: Be cautious when running `rm -rf`

If BUILDDIR were an empty string, running `make clean` would result in
running `rm -rf /*`. Omitting the trailing /* prevents this.

With a valid BUILDDIR, the behaviour of `make clean` changes slightly in
that BUILDDIR itself is removed. However, this is probably more in line
with what one would expect from `make clean`.

Change-Id: I51b52bb6e7fe73a07fed6291a4f1cc253f2bf319
Signed-off-by: Tristan Corrick <tristan@corrick.kiwi>
Reviewed-on: https://review.coreboot.org/27775
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Stefan Tauner <stefan.tauner@gmx.at>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

Documentation/Makefile.sphinx

@@ -46,7 +46,7 @@ help:
 
 .PHONY: clean
 clean:
-	rm -rf $(BUILDDIR)/*
+	rm -rf $(BUILDDIR)
 
 .PHONY: html
 html: