Add following commands to fs shell:
- rf <filename>
- read <filename> [<offset> [<length>]]
- write <filename> [-o <offset>] <byte1> [<byte2> ... [<byteN>]]
- trunc <filename> [<length>]
For writes without offset specified, new data is appended to file.
For truncaces without length specified, 0 is default value.
Signed-off-by: Andrzej Kaczmarek <andrzej.kaczmarek@codecoup.pl>