tools: don't clean when building

Don't depend on clean when building, as the user is capable of cleaning
if required and this introduces a race where "all" depends on both the
compile and the clean in parallel.  It's quite possible for some of the
compile to happen in parallel with the clean, which results in the link
failing as objects just built are missing.

Change-Id: I710711eea7483cafa13251c5d94ec693148bd001
Signed-off-by: Ross Burton <ross.burton@arm.com>