Move build number file to out/soong

Move the build number file to out/soong so Soong can treat it
as an output path, and pass the name of the file to Soong instead
of the command to read it.

Test: m checkbuild
Change-Id: I9e1aecf33d11fe4fde34cd57cc9b1f1ad49b0424
diff --git a/core/config.mk b/core/config.mk
index 844d7d6..ba77990 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -820,7 +820,7 @@
   MAINLINE_SEPOLICY_DEV_CERTIFICATES := $(dir $(DEFAULT_SYSTEM_DEV_CERTIFICATE))
 endif
 
-BUILD_NUMBER_FROM_FILE := $$(cat $(OUT_DIR)/build_number.txt)
+BUILD_NUMBER_FROM_FILE := $$(cat $(SOONG_OUT_DIR)/build_number.txt)
 BUILD_DATETIME_FROM_FILE := $$(cat $(BUILD_DATETIME_FILE))
 
 # SEPolicy versions
diff --git a/core/main.mk b/core/main.mk
index 34d185a..dc8080d 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -38,11 +38,13 @@
 # Write the build number to a file so it can be read back in
 # without changing the command line every time.  Avoids rebuilds
 # when using ninja.
-$(shell mkdir -p $(OUT_DIR) && \
-    echo -n $(BUILD_NUMBER) > $(OUT_DIR)/build_number.txt)
-BUILD_NUMBER_FILE := $(OUT_DIR)/build_number.txt
+$(shell mkdir -p $(SOONG_OUT_DIR) && \
+    echo -n $(BUILD_NUMBER) > $(SOONG_OUT_DIR)/build_number.txt)
+BUILD_NUMBER_FILE := $(SOONG_OUT_DIR)/build_number.txt
 .KATI_READONLY := BUILD_NUMBER_FILE
 $(KATI_obsolete_var BUILD_NUMBER,See https://android.googlesource.com/platform/build/+/master/Changes.md#BUILD_NUMBER)
+$(BUILD_NUMBER_FILE):
+	touch $@
 
 DATE_FROM_FILE := date -d @$(BUILD_DATETIME_FROM_FILE)
 .KATI_READONLY := DATE_FROM_FILE
diff --git a/core/soong_config.mk b/core/soong_config.mk
index f31c9a0..abe1923 100644
--- a/core/soong_config.mk
+++ b/core/soong_config.mk
@@ -25,7 +25,7 @@
 $(call add_json_str,  Make_suffix, -$(TARGET_PRODUCT))
 
 $(call add_json_str,  BuildId,                           $(BUILD_ID))
-$(call add_json_str,  BuildNumberFromFile,               $(BUILD_NUMBER_FROM_FILE))
+$(call add_json_str,  BuildNumberFile,                   build_number.txt)
 
 $(call add_json_str,  Platform_version_name,             $(PLATFORM_VERSION))
 $(call add_json_val,  Platform_sdk_version,              $(PLATFORM_SDK_VERSION))