Makefile: Remove config-devices.mak on "make clean"

The config-devices.mak files are generated by "make", and so they
should be deleted by "make clean".

(This is different from config-host.mak and config-all-disas.mak,
which are created by "configure" and so only deleted by
"make distclean".)

If we don't delete these files on "make clean", then the build
tree is left in a state where it has the config-devices.mak
file but not the config-devices.mak.d file, and make will not
realize that it needs to rebuild config-devices.mak if, for
instance, hw/sd/Kconfig changes.

NB: config-all-devices.mak is also generated by "make", but we
already remove it on "make clean".

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20200717152508.10272-1-peter.maydell@linaro.org