Makefile dependencies for device configs

Add makefile dependencies for target specific device configs.
These will copy the default config if none exists, obsoleting the old
configure time code.  If a config already exists but is older than the
default then print a warning.

Also remove config-devices.h.  Code does not and should not care which
devices are being built.

Signed-off-by: Paul Brook <paul@codesourcery.com>