I want to change the semantics of the CONFFILES variable and improve its handling during the packaging process. Please see detailed reasons and design bekow.

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For now, the CONFFILES variable is a list of config files to be
packaged. However, as these files may come directly from upstream
instead of from our WORKDIR, it's a common mistake that we forget to set
this variable in our recipes. This will lead to unexpected behavior
when doing an on-target upgrade. For example, we modify the /etc/login.defs
on target and then do an upgrade. The file will be replaced, which is
obviously not correct.

We should change the handling and semantics of CONFFILES.
The CONFFILES should be able to take the same form as FILES.
That means, we don't have to list a bunch of files for CONFFILES in each recipe. It will just be expanded like the FILES variable.

As almost all files under /etc are basically configuration files, we should
provide a default value for CONFFILES.

	CONNFFILES_DEFAULT = "${sysconfdir}"

In this way, we don't need to modify every recipe to set the CONFFILES
variable. Of course, setting CONFFILES in recipes take precedence over
the CONFFILES_DEFAULT. For example, if the recipe author decides that
package A should only treat files under ${sysconfdir}/default/ as config
files, he/she can write like this.

       CONFFILES_A = "${sysconfdir}/default"

The above situation should not be common. As according to FHS, the /etc
directory is a place for all system related configuration files.

//Chen Qi