configure: Avoid error messages about missing *-config-*.h files

When compiling with --disable-system there is a harmless yet still
annoying error message at the end of the "configure" step:

 sed: can't read *-config-devices.h: No such file or directory

When only building the tools or docs, without any emulator at all,
there is even an additional message about missing *-config-target.h
files.

Fix it by checking whether any of these files are available before
using them.

Fixes: e0447a83 ("configure: Poison all current target-specific #defines")
Reported-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210519113840.298174-1-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>