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: NMichael Tokarev <mjt@tls.msk.ru>
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20181109161352.29873-1-mdroth@linux.vnet.ibm.com
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
(cherry picked from commit 3fccd3f2)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

scripts/make-release

@@ -19,6 +19,7 @@ pushd ${destination}
 git checkout "v${version}"
 git submodule update --init
 (cd roms/seabios && git describe --tags --long --dirty > .version)
+(cd roms/skiboot && ./make_version.sh > .version)
 # FIXME: The following line is a workaround for avoiding filename collisions
 # when unpacking u-boot sources on case-insensitive filesystems. Once we
 # update to something with u-boot commit 610eec7f0 we can drop this line.