bitbake.conf: set CONFFILES by default

bitbake.conf: set CONFFILES by default

There are a lot of binary packages have files in /etc/, but only a few
of them have set CONFFILES, more than 180 packages are not set in a
world build.  So treat all the files in /etc/ as CONFFILES as Debian
does:
- All the files in /etc/ are CONFFILES
- Move the file out of /etc/ if it is not a conffile
- If /etc/foo can't be moved out, and is changed during
  runtime, then create a symlink /etc/foo -> /var/foo

[YOCTO #8436]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>