disas: Add capstone as submodule

Do not require the submodule, but use it if present.  Allow the
command-line to override system or git submodule either way.

Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

.gitmodules

@@ -37,3 +37,6 @@
 [submodule "ui/keycodemapdb"]
 	path = ui/keycodemapdb
 	url = git://git.qemu.org/keycodemapdb.git
+[submodule "capstone"]
+	path = capstone
+	url = git://git.qemu.org/capstone.git