Check all missing tools at once, fix #1046

Instead of bailing out when detecting a missing tool, this patch
adjusts tool_chain to check for all tools first and then terminate
with an error message afterwards. This makes installing necessary
software easier as you only need to run the script once to get a
list of all missing tools.
Merge branch 'issue1046' of github.com:bjoernd/genode into issue1046