Check the toolset dependencies only if toolset is not installed

[eugulixes]

Currently Pieman checks the toolset dependencies even if toolset is installed. For example,

Info: checking Mender dependencies                                                                                                   
checking /usr/include/sys/types.h... yes                                                                                             
checking bc... yes                                                                                                                   
checking cc... yes                                                                                                                   
checking dtc... yes                                                                                                                  
checking go... yes                                                                                                                   
checking make... yes

It can be confusing.