modify romfs.c to use the new (standard) romfs format

the change is mainly done in adding romfs_get_file_by_name ,
which supports referencing file given a path. the file system no longer
uses hash.
a file size field is added to romfs_fds ,since new file header includes
file name ,which is variable length ,making directly referencing size
field from content pointer more difficult
fix dir spec.info in mkromfs & filesystem warning

when making directory in mkromfs ,forgot to add filename offset to
spec.info (points to files in directory)