doc: make docs build conditionally
doc: make docs build conditionally

Currently docs will build the images regardless of the setting for
enable-user-docs at configure time.
The image generation should be dependent on actually building docs so
that systems without the image tools installed will not generate
warnings.

Signed-off-by: Mike Holmes <ed44c09f4d7f698f5510adc894a872c73e08c8bd@linaro.org>
Reviewed-by: Anders Roxell <b4e8e89057bc64bc4f9625a4a821c9ac177a7ed2@linaro.org>
Signed-off-by: Maxim Uvarov <db4d16e02ae2d7493db430203537da8b2e34f290@linaro.org>

dkrot/odp,erachmi/odp,mike-holmes-linaro/odp,mike-holmes-linaro/odp,rsalveti/odp,erachmi/odp,nmorey/odp,ravineet-singh/odp,dkrot/odp,rsalveti/odp,rsalveti/odp,erachmi/odp,dkrot/odp,rsalveti/odp,erachmi/odp,rsalveti/odp,dkrot/odp,ravineet-singh/odp,nmorey/odp,nmorey/odp,ravineet-singh/odp,mike-holmes-linaro/odp,ravineet-singh/odp,nmorey/odp,mike-holmes-linaro/odp