Build gaia into /data as part of build process (and generate userdata.img)

[joneschrisg]

The current system is incredibly fragile. We need to set up $OUT/data properly during the build so that

• userdata.img gets generated properly
• fastboot flashall -w works correctly

This is very high priority.

[joneschrisg]

@fabricedesre is integrating some work from @vingtetun that should help solve this problem.