More improvements to the shell (WIP)

- file masks ('*' and '?' are now accepted)
- recursive operations for ls/cp
- new FS-specific functions (src/common_fs.c)
added MMCFS emulation under the simulator
Adding rm and rmdir commands to the shell (WIP)
'mv' command and shell code refactoring

There is a new command in the shell ('mv' - move/rename files).
Also, the shell code was refactored into its own directory (src/shell).
The 'help' command in the shell was updated and supports 'help <command>'.
There is a new BUILD_ADVANCED_SHELL build time macro that enables the
advanced shell features (currently 'cp' with recursion (and other improvements),
'mv' and 'rm').
shell fix for shell_recv
Shell commands are now callable from the 'elua' module

Simply use 'elua.shell( <command> )', for example:

elua.shell( "ls /rom" )

In called like this, "lua" and "exit" are disallowed.
don't include cmn_fs* functions in basic shell mode
fixed invalid return value in cmn_fs_get_type
small style changes
fixed potential double dm_closedir calls
Working on the documentation for the new shell commands

- "using.html" converted to AsciiDoc mode
- the shell documentation was split in a separate file and divided into
  two parts: docs for the simple shell and for the advanced shell
  (yet to be written)
improved file type reporting
added documentation about the advanced shell
fixed to shell code in TCP/IP mode
fix scons compilation with the new shell
Merge branch 'shellfs'