doc: General makefile cleanup.

This is a general cleanup of the makefile in order to bring it more inline
with standard make practices. Cleanups included more use of automatic
variables, switching AM_V_GEN to AM_V_at to have one 'GEN' visible per file,
splitting copy operations to proper rules, and using order only dependencies
to properly create directories on-demand.

Changes also correct missing use of $(builddir) that has gone unnoticed as
it defaults to the current directory ('.').

Signed-off-by: Jon A. Cruz <jonc@osg.samsung.com>
Acked-by: Peter Hutterer <peter.hutterer@who-t.net>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>