docs/manual: copying the manual sources is a common action

Move the creation of $(BUILD_DIR)/manual and the rsyncing of the
sources into GENDOC.

This is is needed so that GENDOC can be easily used to generate another
document, without requiring that document to duplicate the copying rules.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Reviewed-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>