tftpd: new applet (mostly using existing code for tftp)

tftpd: new applet (mostly using existing code for tftp)

function                                             old     new   delta
tftp_protocol                                          -    1173   +1173
tftpd_main                                             -     500    +500
tftp_option_get                                        -     102    +102
packed_usage                                       23650   23662     +12
applet_names                                        1809    1815      +6
applet_main                                         1100    1104      +4
applet_nameofs                                       550     552      +2
tftp_main                                            302     301      -1
get_nport                                             32       -     -32
tftp                                                1172       -   -1172
------------------------------------------------------------------------------
(add/remove: 3/2 grow/shrink: 4/1 up/down: 1799/-1205)        Total: 594 bytes
   text    data     bss     dec     hex filename
 796479     662    7420  804561   c46d1 busybox_old
 797153     662    7420  805235   c4973 busybox_unstripped