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

Bug: 36726045
Test: manual with hikey
Test: boot and init tests on bullhead
Test: cast with fugu, per b/36726045
Merged-In: If3c30a2fff58cfece2fcd27e69c30382146e6808

Change-Id: If3c30a2fff58cfece2fcd27e69c30382146e6808
Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>