Add TARGET_RECOVERY_FSTAB_GENRULE

This variable can be set instead of TARGET_RECOVERY_FSTAB
in order to specify a generated fstab file.

Bug: 201700692
Test: Presubmits
Change-Id: If0fae81ec53915e9ed4bab2e8b1a9f8376894537
diff --git a/core/Makefile b/core/Makefile
index 26ed864..c7e60f9 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -2132,6 +2132,9 @@
 
 ifdef TARGET_RECOVERY_FSTAB
 recovery_fstab := $(TARGET_RECOVERY_FSTAB)
+else ifdef TARGET_RECOVERY_FSTAB_GENRULE
+# Specifies a soong genrule module that generates an fstab.
+recovery_fstab := $(call intermediates-dir-for,ETC,$(TARGET_RECOVERY_FSTAB_GENRULE))/$(TARGET_RECOVERY_FSTAB_GENRULE)
 else
 recovery_fstab := $(strip $(wildcard $(TARGET_DEVICE_DIR)/recovery.fstab))
 endif