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.

(cherry-pick of 5a88269a.)

Bug: 7253452
Change-Id: I21eb93f3cb430c9531fe41a2f5d7b445c09938b9