UBUNTU: [Packaging] re-introduce linux-doc as an empty package

In commit fd8ee93c3a38 ("UBUNTU: [Packaging] drop useless linux-doc") we
decided to remove the linux-doc package, saving in this way a lot of
space required to produce the documentation content (that can be easily
found online).

However, this change broke some user-space packages that are still
requiring linux-doc as a dependency.

Therefore, re-introduce an empty linux-doc package to prevent the
breakgage and at the same time avoid to re-introduce the expensive build
dependencies.

Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
[esmil: handle linux-doc like the linux-source package]
Signed-off-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>

debian/control.d/linux-doc.stub

@@ -0,0 +1,8 @@
+Package: linux-doc
+Build-Profiles: <!stage1>
+Architecture: all
+Section: doc
+Priority: optional
+Description: Linux kernel specific documentation for version PKGVER
+ This package is deprecated and it is temporarily provided only for
+ compatibility reasons. It will be dropped in the future.

debian/rules

@@ -173,6 +173,7 @@ ifeq ($(src_pkg_name),linux)
 	control_files += debian/control.d/linux-cloud-tools-common.stub
 	control_files += debian/control.d/linux-tools-host.stub
 	control_files += debian/control.d/linux-source.stub
+	control_files += debian/control.d/linux-doc.stub
 endif
 
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