Build otacerts as a module.

Bug: 30414428
Test: `m -j dist` with aosp_taimen-userdebug. Check
      /system/etc/security/otacerts.zip available under system and
      recovery images.
Change-Id: I5abeb2da441fb3e3231e094063c2383eb3807852
(cherry picked from commit b7936f3d2e16bcad36dcbfc72123381726dbc3c5)
diff --git a/target/product/base_vendor.mk b/target/product/base_vendor.mk
index 584327c..c27407a 100644
--- a/target/product/base_vendor.mk
+++ b/target/product/base_vendor.mk
@@ -23,6 +23,7 @@
     init_second_stage.recovery \
     ld.config.recovery.txt \
     linker.recovery \
+    otacerts.recovery \
     recovery \
     shell_and_utilities_recovery \
     watchdogd.recovery \