kbuild: allow docproc invocation from external

- getcwd returns path without a slash at the end, add the slash
- add KBUILD_SRC env support, so that we can specify path for
  kernel (to know where scripts/kernel-doc resides) and SRCTREE
  (for searching files referenced in .tmpl) separately

[v2]
- use KBUILD_SRC instead of a newly introduced environment variable

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>