make-release: add skiboot .version file

make-release: add skiboot .version file

This is needed to build skiboot from tarball-distributed sources
since the git data the make_release.sh script relies on to generate
it is not available.

Cc: qemu-stable@nongnu.org
Reported-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20181109161352.29873-1-mdroth@linux.vnet.ibm.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
(cherry picked from commit 3fccd3f26ef3c0c76a06c138b17af6d55a5d9904)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>