nshlib: Support "-f" option for command "rm"

In below two cases "rm" command with "-f" option will return 0:
  a. Bad arguments
  b. File not exists

Following "rm" of GNU coreutils 8.32

GNU coreutils
  $ rm --version
  rm (GNU coreutils) 8.32
  Copyright (C) 2020 Free Software Foundation, Inc.
  License GPLv3+: GNU GPL version 3 or later <https://gnu.org/licenses/gpl.html>.
  This is free software: you are free to change and redistribute it.
  There is NO WARRANTY, to the extent permitted by law.

  Written by Paul Rubin, David MacKenzie, Richard M. Stallman,
  and Jim Meyering.

  $ rm /FILE_NOT_EXISTS
  rm: cannot remove '/FILE_NOT_EXISTS': No such file or directory
  $ echo $?
  1

  $ rm -f
  $ echo $?
  0

  $ rm -f /FILE_NOT_EXISTS
  $ echo $?
  0

Without this patch
  nsh> rm
  nsh: rm: missing required argument(s)

  nsh> rm /FILE_NOT_EXISTS
  nsh: rm: unlink failed: 2
  nsh> echo $?
  1

With this patch
  nsh> rm -f
  nsh> echo $?
  0

  nsh> rm -f /FILE_NOT_EXISTS
  nsh> echo $?
  0

Signed-off-by: wangjianyu3 <wangjianyu3@xiaomi.com>

nshlib/nsh_command.c

@@ -505,7 +505,7 @@ static const struct cmdmap_s g_cmdmap[] =
 
 #ifdef NSH_HAVE_DIROPTS
 #  ifndef CONFIG_NSH_DISABLE_RM
-  CMD_MAP("rm",       cmd_rm,       2, 3, "[-r] <file-path>"),
+  CMD_MAP("rm",       cmd_rm,       2, 3, "[-rf] <file-path>"),
 #  endif
 #endif