[PATCH] RSB: add GRUB2 to the RTEMS tools and use it in 6/rtems-x86_64 build set

[Karel Gardas]

This patch is tested on Ubuntu 20.04, FreeBSD 13 and Mac OS X Catalina.
It is not tested on Windows due to IMHO current documentation for
Windows host to be either out-dated or not correct and I'm not able to
follow it to even build RSB without the patch.

Yes, on FreeBSD 13 the patch is tested after applying dtc 1.6.1 update
patch to unbreak FreeBSD build first.

Karel