[doit] Switch to using args rather than env vars
[doit] Switch to using args rather than env vars

Now to invoke doit rather than:
ARCHES='arm sh' FETCH=1 ./doit
you do:
./doit -a 'arm sh' -f

There are also flags for using CCACHE and make parallelization:
./doit -a 'arm' -f -j64 -c

travisg/toolchains