scr_switch_debugger.rb - Yast is not required at the target system

y2base binary and Yast packages might be missing so only glibc seems
to be really needed
require `yast2-core` and `perl-bootloader` instead of `yast2-installation`

for now need at least these two packages, but should be removed later completely
Merge pull request #70 from yast/no_yast_packages

No yast packages in the target