Build only modules required by the current product.

Don't build modules_to_check by default.
Instead add "checkbuild" to the command line as a build goal, if you
want to build everything.
We can use the "checkbuild" goal to make sure some build targets on the
build server still build everything.

Bug: 7253452
Change-Id: I84b62116131e85d1f32d58a424cdfd9d51fa9755