Create a common BR2_COPYTO to prepare to replace

Create a common BR2_COPYTO to prepare to replace
all the different COPYTOs spread out all over
Buildroot.
Generate COPY_FILE subroutine which can be used
to copy a file.

$(COPY_FILE , file, directory, filename)
This will copy to binary dir and to the selected
copy dir if set in new config (intended to replace
all old COPYTO configs)

This is overridden by BUILDROOT_COPYTO,
if set into the environment.