Move BUILD_*_FROM_FILE to config.mk

BUILD_NUMBER_FROM_FILE and BUILD_DATETIME_FROM_FILE may need to
be referenced before main.mk has been parsed, move them to config.mk.

Bug: 68865363
Test: m -j TARGET_PRODUCT=full TARGET_BUILD_APPS=Gallery
      aapt dump xmltree out/target/product/generic/obj/APPS/Gallery_intermediates/package.apk AndroidManifest.xml | grep Version
Change-Id: I9361a394469e61fcdf5a2b9509ef2bb2b373d6e7
diff --git a/core/config.mk b/core/config.mk
index dca7289..f7d5ab8 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -741,6 +741,9 @@
   FRAMEWORK_COMPATIBILITY_MATRIX_FILE := hardware/interfaces/compatibility_matrix.current.xml
 endif
 
+BUILD_NUMBER_FROM_FILE := $$(cat $(OUT_DIR)/build_number.txt)
+BUILD_DATETIME_FROM_FILE := $$(cat $(OUT_DIR)/build_date.txt)
+
 # ###############################################################
 # Set up final options.
 # ###############################################################