unify help string style

- Add help texts for many commands.
- Let the linker sort the command table.
- Add support for multiple argmuments in several commands
  (mkdir, rmdir, rm, cat)

- do more POSIX:
  - use DIR instead of struct dirent
  - use (struct dirent)->d_name instead of (struct dirent)->name
- switch to a new layout for U_BOOT_CMD:
  - use C99 initializers to be able to add more fields to the
    command struct
  - add aliases for commands (needed mainly for help -> ? and test -> [
  - This is not done for all commands yet, but the compiler will tell you ;)