Fix shellcheck issue in firstboot script

Before this change, the firstboot script was not
passing shellcheck test as it invoked jffs2reset
without quotes, leading to potential re-splitting.

Signed-off-by: Matteo Iervasi <matteo.iervasi@edalab.it>

package/base-files/files/sbin/firstboot

@@ -1,3 +1,3 @@
 #!/bin/sh
 
-/sbin/jffs2reset $@
+/sbin/jffs2reset "$@"