Fix tar error when packing files

Multiple quoted files are treated by tar as one file.
Test: ran `./gen-bios-tar -v "rev1" -m "MyTestMachine" bios.bin`

Signed-off-by: Glukhov Mikhail <mikl@greenfil.ru>
Change-Id: I1a0ad1d0dd89d8830a50fd059acac285156f04da
diff --git a/gen-bios-tar b/gen-bios-tar
index 837d496..2890cb3 100755
--- a/gen-bios-tar
+++ b/gen-bios-tar
@@ -167,5 +167,8 @@
   additional_files="*.sig"
 fi
 
-tar -czvf "$outfile" "$files_to_sign" "$additional_files"
+# shellcheck disable=SC2086
+# Do not quote the files variables since they list multiple files
+# and tar would assume to be a single file name within quotes
+tar -czvf $outfile $files_to_sign $additional_files
 echo "Bios image tarball is at $outfile"