image_types.bbclass: manage 'cpio_append' directory

image_types.bbclass: manage 'cpio_append' directory

For cpio images, do_rootfs() can operate on a dirty '${WORKDIR}/cpio_append'
directory which contains e.g. files from previous builds.  This can cause
unwanted files in the image or can break the build.

E.g. when there is a cpio_append/init -> /sbin/init symlink symlink, the
'ln -sf' can fail due to SELinux restrictions:

| $ ls -la cpio_append/init
| lrwxrwxrwx. 1 ensc ensc 10 22. Jan 16:26 cpio_append/init -> /sbin/init
|
| $ strace ln -sf /sbin/init cpio_append/init
| ...
| stat("cpio_append/init", 0x7fffbb9ca310) = -1 EACCES (Permission denied)
| exit_group(1)                           = ?

Patch cleans up 'cpio_append' before executing the 'do_rootfs' task by
adding it to 'cleandirs'.  An alternative implementation (which avoids
creation of this empty dir for non-cpio images) might remove it within
IMAGE_CMD_cpio, but this might break builds where people rely on the
existence of this directory (e.g. to add local files).

Signed-off-by: Enrico Scholz <enrico.scholz@sigma-chemnitz.de>
Signed-off-by: Ross Burton <ross.burton@intel.com>