Refactor some scripts

[alexeykuzmin]

No breaking changes (with exception of renamed --target-arch → --target_arch arg of the apply-patches script).

• Move some script constants to a shared "lib/config.py"
• Add and use git.get_head_commit()
• Extract Ninja related stuff to "lib/ninja.py"
• Add 'run-gn' script and call it from the 'update' script
• Name an arg "target_arch" instead of "target-arch" for consistency

[alexeykuzmin]

@nornagon Any other suggestions to make it even better? Did I forget to change anything?

[alexeykuzmin]

@ckerr, @nornagon Thanks for the feedback. I'll try to take it into account on the next iteration )