support new build_dir variable in documents

We do not build the documents in other directories than the source
directory. So we need to use the build_dir variable to refer to the
source directory in the documents.

Author: jnavila

makefile.locale

@@ -54,7 +54,7 @@ ASCIIDOC_EXTRA += -amanmanual='Git Manual' \
                   -amansource='Git $(GIT_VERSION)' \
                   -amanversion=$(GIT_VERSION)
 ASCIIDOC_EXTRA += -alitdd='\--'
-ASCIIDOC_EXTRA += -acompat-mode -atabsize=8
+ASCIIDOC_EXTRA += -acompat-mode -atabsize=8 -abuild_dir=.
 
 $(HTML_FILTER): %.html: %.adoc
 	$(QUIET_ASCIIDOC)asciidoctor -b xhtml5 \