The following patch fixes up 'htmldocs' and related to work when
trees are being built with O=.  I fixed it all up by passing the srctree
as an env-var to docproc (and thus what it calls) and then pull that out
when needed.

Signed-off-by: Tom Rini <trini@kernel.crashing.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>