tools: fix clean rule in tool_chain_rump

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.
tools: strip rumpkernel based tools

Until now, the rumpkernel based tools were installed with all symbols
included. This accounts for about 200MiB used space on 64Bit system.
Stripping the binaries prior to installation brings the space
requirements down to 20MiB.

Fixes #1245.