Some corrections and improvements from testing on my side

[dmikushin]

Hi @vishalbiswas thanks for your great work on hacking into Google's comprehensive build system. I was able to produce valid ARM binaries using your script. However, I get Ninja complaints about missing zlib files, unless I re-enable it in the script config. Also, it should be better not to delete existing repos, and only update them. Finally, I have added some info into the README.

[vishalbiswas]

Thanks for the changes. I will take a look later today.

[vishalbiswas]

Turns out I had some code for this in working directory for more than 2 years.
Please let me know if the issue persists in latest push.

Also, after you've downloaded the whole thing once, you can:

1. Set skipindeps=yes and skipsrc=yes, if you use the same buildref.
2. Set skipcross=yes, if you use the same arch.

For both downloads (tarballs and git), the script only downloads sources for buildref instead of cloning whole repository.