The clean rule is used to delete already built binaries as well as to
clean-up any left-overs from previous build attempts. If there was
no previous attempt just return true to prevent make from complaining.
Fixes#1245.
The rumpkernel tools are used within the Genode OS Framework tool chain
for preparing and populating disk images as well as creating cgd(4)
based cryptographic disk devices.
Execute 'tool/tool_chain_rump build' to build the tools and afterwards
'tool/tool_chain_rump install' to install the binaries. The default
install location is _/usr/local/genode-rump_.