git.sh: make git_prep command clearer

the "u" argument can actually be any thing. git_prep
handles git submodules only for single-tree projects,
under any candition, or on multi-tree projects if
the number of arguments to git_prep is above four.

"u" is the 5th argument, meant to enable submodule
downloads. it really doesn't matter what this string
says, so let's just make it as clear as possible.

Signed-off-by: Leah Rowe <leah@libreboot.org>

include/git.sh

@@ -15,7 +15,7 @@ fetch_targets()
 
 	printf "Creating %s tree %s\n" "$project" "$tree"
 	git_prep "$loc" "$loc" "$xbmkpwd/$configdir/$tree/patches" \
-	    "src/$project/$tree" u
+	    "src/$project/$tree" with_submodules
 	nuke "$project/$tree" "$project/$tree"
 }