This command can return the .config file to the user. It can be useful
to trace what is actually running, as addition to the "ver"
command.  It is optional because it costs .5kB of binary size.

Example:

  wrc# config
    Current WRPC-SW configuration:
  CONFIG_RAMSIZE=90112
  # CONFIG_PTP_NOPOSIX is not set
  CONFIG_PPSI=y
  # CONFIG_CHECK_RESET is not set
  CONFIG_PPSI_VERBOSITY=0
  CONFIG_PPSI_RUNTIME_VERBOSITY=y
  CONFIG_STACKSIZE=2048
  CONFIG_PP_PRINTF=y
  # CONFIG_PRINTF_FULL is not set
  CONFIG_PRINTF_XINT=y
  # CONFIG_PRINTF_MINI is not set
  # CONFIG_PRINTF_NONE is not set
  CONFIG_PRINT_BUFSIZE=128
  CONFIG_ETHERBONE=y
  # CONFIG_DETERMINISTIC_BINARY is not set
  CONFIG_CMD_CONFIG=y

(This configuration amounts to a binary size of 86252 bytes, at this
commit).
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>