default empty tapas to TARGET_BUILD_APPS "all"

So that "tapas" without argument won't end in makefile-could-not-load error in unbundled branch.
Also, the magic "all" apps will build only apps, not other things.

Change-Id: Iac6dd928ee4e42996147acdab31930cd4ffa1768
diff --git a/core/main.mk b/core/main.mk
index ddd05bb..ae7abb6 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -692,8 +692,8 @@
 
   unbundled_build_modules :=
   ifneq ($(filter all,$(TARGET_BUILD_APPS)),)
-    # If they used the magic goal "all" then build everything
-    unbundled_build_modules := $(sort $(call get-tagged-modules,$(ALL_MODULE_TAGS)))
+    # If they used the magic goal "all" then build all apps in the source tree.
+    unbundled_build_modules := $(foreach m,$(sort $(ALL_MODULES)),$(if $(filter APPS,$(ALL_MODULES.$(m).CLASS)),$(m)))
   else
     unbundled_build_modules := $(TARGET_BUILD_APPS)
   endif
@@ -786,4 +786,3 @@
 .PHONY: showcommands
 showcommands:
 	@echo >/dev/null
-