Split out base squashfs.exclude

Variants no longer need to specify the full contents of the
`squashfs.exclude` file.  The "base" file in the source directory gets
combined with the one from the config directory (if it exists).

squashfs.exclude

@@ -0,0 +1,56 @@
+*/*/*/*/.keep_*
+*/*/*/.keep_*
+*/*/.keep_*
+*/.keep_*
+bin/bb
+etc/X11
+etc/conf.d
+etc/csh.env
+etc/init.d
+etc/portage
+etc/python-exec
+etc/runlevels
+etc/selinux/semanage.conf
+etc/selinux/sepolgen.conf
+usr/bin/2to3*
+usr/bin/audit2*
+usr/bin/chcat
+usr/bin/idle
+usr/bin/pydoc*
+usr/bin/python*
+usr/bin/pyvenv
+usr/bin/rlpkg
+usr/bin/semanage
+usr/bin/sepolgen*
+usr/bin/sepolicy
+usr/include
+usr/lib*/*.a
+usr/lib*/cmake
+usr/lib*/libpython*
+usr/lib*/pkgconfig
+usr/lib/kernel
+usr/lib/python*
+usr/lib/rpm
+usr/lib/udev/hwdb.d
+usr/local
+usr/share/aclocal
+usr/share/baselayout
+usr/share/bash-completion
+usr/share/doc
+usr/share/factory
+usr/share/gdb
+usr/share/info
+usr/share/locale/*/*/*python*
+usr/share/man
+usr/share/pkgconfig
+usr/share/polkit-1
+usr/share/portage
+usr/share/selinux/*/include
+usr/share/selinux/devel
+usr/share/zsh
+var/.updated
+var/cache/edb
+var/db/Makefile
+var/db/pkg
+var/lib/portage
+var/log/journal