cp: add -u/--update and --remove-destination

Based on the patch by wdlkmpx@gmail.com

function                                             old     new   delta
copy_file                                           1546    1644     +98
add_partition                                       1270    1362     +92
ask_and_unlink                                        95     133     +38
do_iproute                                           132     157     +25
decode_one_format                                    710     715      +5
cp_main                                              369     374      +5
ubirename_main                                       198     202      +4
read_package_field                                   232     230      -2
bb_make_directory                                    421     412      -9
packed_usage                                       30505   30476     -29
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 7/3 up/down: 267/-40)           Total: 227 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
3 files changed