doc: Fix build and installation of yat2m.

* configure.ac (YAT2M): Require it when cross compiling.
* doc/Makefile.am (bin_PROGRAMS): Add yat2m.

Before 1.28, yat2m used to be an internal tool. Now, it's a standard
tool. For native build, it is built and used for initial
installation. For cross build, it is required on the build system.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>