gendoc infra: avoid a2x warning

Though the --destination-dir option works as expected, a2x displays the
following message when generating the pdf and text manual:

  a2x: WARNING: --destination-dir option is only applicable to HTML based outputs

To avoid this warning, we now just build the manual in its build location,
then move the generated files into $(O)/docs/manual.

Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
[yann.morin.1998@free.fr: tested all but PDF]
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>