print_toolchain_versions: print system and make's default shell.
print_toolchain_versions: print system and make's default shell.

Some systems use dash as system shell, others use bash. The shell used
by make can also be different, and unrelated to the system shell.
Differences in this variable can cause problems when testing PRs and
reporting bugs.

The default shell is important system information that should be reported.

mtausig/RIOT,miri64/RIOT,kYc0o/RIOT,ant9000/RIOT,toonst/RIOT,yogo1212/RIOT,x3ro/RIOT,mtausig/RIOT,kYc0o/RIOT,kYc0o/RIOT,x3ro/RIOT,miri64/RIOT,RIOT-OS/RIOT,basilfx/RIOT,miri64/RIOT,x3ro/RIOT,ant9000/RIOT,miri64/RIOT,toonst/RIOT,authmillenon/RIOT,ant9000/RIOT,toonst/RIOT,yogo1212/RIOT,smlng/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,OTAkeys/RIOT,toonst/RIOT,x3ro/RIOT,jasonatran/RIOT,kaspar030/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,kYc0o/RIOT,OlegHahm/RIOT,smlng/RIOT,basilfx/RIOT,kaspar030/RIOT,ant9000/RIOT,OlegHahm/RIOT,miri64/RIOT,smlng/RIOT,yogo1212/RIOT,jasonatran/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,OTAkeys/RIOT,kaspar030/RIOT,jasonatran/RIOT,smlng/RIOT,mtausig/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,ant9000/RIOT,authmillenon/RIOT,OlegHahm/RIOT,basilfx/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,kaspar030/RIOT,mtausig/RIOT,basilfx/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,kYc0o/RIOT,smlng/RIOT,yogo1212/RIOT,x3ro/RIOT,mtausig/RIOT,basilfx/RIOT,authmillenon/RIOT,kaspar030/RIOT,authmillenon/RIOT,jasonatran/RIOT,jasonatran/RIOT,toonst/RIOT