fix doc-build.sh to support referring to other dirs

fix doc-build.sh to support referring to other dirs

In [1], we found that current doc-build.sh cannot create document
when the main index.rst referring to rst file in another folder.
The reason is current script only copies target src folder under the
working directory.

This patch fix this bug by making doc-build.sh to copy all src files
in the original directory structure. Then, all file links (import)
can work properly.

Error message for '_sha1_' is modified to drop directory info, but
developer can find a bad file path from grep output.

[1] https://gerrit.opnfv.org/gerrit/#/c/4353

Change-Id: I1d840b8a887306539f97321570f289ac25055a88
Signed-off-by: Ryota MIBU <r-mibu@cq.jp.nec.com>