Add find, tar, bzip2 to RosBE required tools list

RosBE-Builder.sh has the following line:

rs_needed_tools="as bison flex $CC $CXX grep makeinfo python"        # GNU Make has a special check

According to neonx a minimal nspawn container does not contain find, tar or bzip2 by default so these should probably be added to the list so we properly complain about them