docs/manual: clarify the reinstallation of a package

docs/manual: clarify the reinstallation of a package

Since <package>-rebuild implies <package>-reinstall and
<package>-reconfigure implies <package>-rebuild, it is confusing
to mention the make and make <package> commands when describing
the restart of the configuration, compilation and installation of
the package.

Therefore remove the ", followed by +make+ or +make <package>+"
portions in the "8.3. Understanding how to rebuild packages" section,
and add a new paragraph clarifying how to include the rebuilt package
in the filesystem image, if that is necessary.

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>