rootfsdebugfiles.bbclass: quick-and-dirty installation of additional files

rootfsdebugfiles.bbclass: quick-and-dirty installation of additional files

The main motivation for this class was the observation that
a) a core-image can hang under qemu when the kernel does not
   have enough entropy to generate the ssh host key
b) ssh complains about changing ssh host key files when
   rebooting the same machine with different images

For debugging it is okay to reuse an ssh host key generated on the device
before. There may be also similar use cases, so the class is generic enough to
also copy more than one file or directory, with dropbear_rsa_host_key given as
example.

The documentation and naming of the class makes it clear that it
should not be used for production images.

(From OE-Core rev: ae4d605d802edb4c8bdb2dbee1b0c36d7730c07f)

Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>