Use Paths.mk for docs, stubdom and tools build

Use Paths.mk for docs, stubdom and tools build

Paths.mk contains just directories set by configure.
Source this file first when building docs,stubdom or tools.
Remove unused path variables from $subsystem.mk.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Samuel Thibault <samuel.thibault@ens-lyon.org>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>