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 5a88269ad6ed44fb163f96847954faac21292567.)

Bug: 7253452
Change-Id: I21eb93f3cb430c9531fe41a2f5d7b445c09938b9
diff --git a/core/main.mk b/core/main.mk
index 7bd55b8..77f9fc5 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -710,7 +710,6 @@
 .PHONY: files
 files: prebuilt \
         $(modules_to_install) \
-        $(modules_to_check) \
         $(INSTALLED_ANDROID_INFO_TXT_TARGET)
 
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