doc: Add the U-Boot logo to the html doc

Now we have a logo for U-Boot, we can include it in the Sphinx
html doc.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/doc/conf.py b/doc/conf.py
index 168c313..0772fb6 100644
--- a/doc/conf.py
+++ b/doc/conf.py
@@ -170,7 +170,7 @@
 
 # The name of an image file (relative to this directory) to place at the top
 # of the sidebar.
-#html_logo = None
+html_logo = '../tools/logos/u-boot_logo.svg'
 
 # The name of an image file (within the static path) to use as favicon of the
 # docs.  This file should be a Windows icon file (.ico) being 16x16 or 32x32