Introduce 'android' as value of 'target-os' feature.

At present, all it does is skips linking to the rt/pthread libraries.
redistribute whitespace around colon
Add .gitignore
Need to bump MAXJOBS to 256.

Having to move to larger smp clusters so need more build
parallelism.
Export BOOST_JAM_TOOLSET to calling script.

This change allows top-level scripts to determine which
toolchain was used by Boost.Build engine bootstrap process.
Top-level script of Boost C++ Libraries will be updated
separately.