add REPL tasks for Gradle: GroovyConsole (GUI) and Groovysh (console)

Conflicts:

	build.gradle
remove Groovysh (console) REPL

We need a dirty trick to make it find jline. I don't like this. Furthermore, the output of Gradle interferes with the REPL. I think we are better off using the GUI.
use gradle-ssh-plugin
replace one instance of sh(...) by project.exec {...}

I think project.exec{...} has the cleaner syntax, but I found out about it too late. I won't change the existing code, for now.
use project.convention instead of project.extensions for HelpersPlugin
Dropbear: add scp support
add artifactory repository and plugin
start adding sftp support for Dropbear
add zlib (but we still need OpenSSL)
use tarTree instead of external tar tool -> doesn't work
move LazyValue to dedicated Groovy file
use external tar (again) because tarTree doesn't work
add incremental build for fetchAndExtractSource tasks
incremental build for zlib
sftp works
use sftp to transfer files to the board

... instead of 'ssh "cat >file" <data'. We still use NFS for the rootfs, of course.
Merge branch 'master' of github.com:BBBSnowball/mehari