testimage: get real os-release file

testimage: get real os-release file

/etc/os-release is a symlink to /usr/lib.
Symlink is retrieved as a dead link which points to nowhere if also the
original file is not accompanying it.
Fetch the real file in addition to this link.

Alternative could be to use "tar -h" (supported also by busybox tar),
however that could lose some important information if links are relevant
for failure analysis.

Signed-off-by: Peter Marko <peter.marko@siemens.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>