setup_dependencies: update bpftool install directions for Ubuntu

Ubuntu 18.04, 19.10 and later versions have bpftool packaged as part as
the linux-tools-common, linux-tools-generic packages (like perf).

Update the instructions accordingly.

Debian still has no package, leave it in a dedicated section.

Signed-off-by: Quentin Monnet <quentin@isovalent.com>

Author: qmonnet

setup_dependencies.org

@@ -162,9 +162,20 @@ On a machine running the Fedora Linux distribution, install package:
  $ sudo dnf install bpftool
 #+end_example
 
-** Packages on Debian/Ubuntu
+** Packages on Ubuntu
+
+Starting from Ubuntu 19.10, bpftool can be installed with:
+
+#+begin_example
+ $ sudo apt install linux-tools-common linux-tools-generic
+#+end_example
+
+(Ubuntu 18.04 LTS also has it, but it is an old and quite limited bpftool
+version.)
+
+** Packages on Debian
 
-Unfortunately, bpftool is not officially packaged for Debian/Ubuntu
+Unfortunately, bpftool is not officially packaged for Debian
 [[https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=896165)][yet]].
 
 However, note that an unofficial