doc: Add reference to U-Boot project in conf.py

With the last update of conf.py the references to U-Boot where replaced by
references to Linux.

Fix the project references in the generated documentation.

Reported-by: Simon Glass <sjg@chromium.org>
Reported-by: Bin Meng <bmeng.cn@gmail.com>
Fixes: 98f01cf7 ("doc: update Kernel documentation build system")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>