Remove BR2_HAVE_DEVFILES

This finally removes the BR2_HAVE_DEVFILES option, that was used to
install/keep development files on target. With the recent migration of
the internal backend to the package infrastructure, we had anyway lost
the ability to build gcc for the target, and install the uClibc
development files on the target.

[Peter: also remove support/scripts/copy.sh]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>