Log Message:
Update sysbuild to 2.8:

Released on 2025-01-01.

* Made the default.conf configuration file not assume a NetBSD host so
  that it is usable, for example, on Mac OS X.  The previous sample
  configuration file has been renamed to NetBSD.conf and is only
  installed when sysbuild is built on NetBSD.

* Changed the default value of NJOBS to be set to the number of CPUs in
  the machine.

* Fixed default value of SYSBUILD_CONFSUBDIR in configure and a related
  pasto in the INSTALL document.

* Fixed handling of flags in Mac OS X 10.9 due to oddities with the
  built-in /bin/sh (ancient bash).

* Fixed path to sysbuild in sysbuild4cron.

* Converted test programs to shtk's unittest library, thus removing the
  dependency on atf-sh.

* Fixed option parsing to detect missing arguments to options.

* Added support to build specific architectures within a machine type.
  This is done via new syntax in the MACHINE variable, which can now
  take pairs like evbarm-aarch64.

* Updated the minimum shtk version to 1.7.