cancel unused argument from romfs

romfs_get_file_by_hash accepts argument len to store file length, but it
is not used by current system. Furthermore, it will make my future
implementation more complex.
remove size check in romfs_get_file_by_hash

Size-0 files should be conceptually permitted.
insert filename section into romfs

This section is a plaintext of filename, necessary for future "ls"
implementation.
implement simple mounting for future "ls"
get file attributes during traversal

fattr.h is planned to contain many related constants in future, so it is
an independent file now.

Conflicts:

	romfs.c
implement "ls"

This version does not consider the consistency of directories shown in
command line and actual starting point. (/ V.S. romfs/)