init: use read_file and write_file to implement do_copy builtin

this will make the implementation more cleaner,
and has error message output when failed on some operations

also add the O_TRUNC flag explicitly for the open function
called in write_file.

And add more test on read_file and write_file functions

Test: manual with hikey

Change-Id: Ifc1086a20e85db6980b497b1150a8a7952e672d6
Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>