exec: don't include hw/boards for linux-user

As noted by Andreas, hw/boards.h shouldn't be used outside softmmu code.
Include it conditionally, and drop the (now unnecessary) ifdef guards in
hw/boards.h

Reported-by: Andreas Färber <afaerber@suse.de>
Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
