build_image: check value of "ext4_share_dup_blocks"

Regardless of the value of "ext4_share_dup_blocks" "-c" flag is passed
unconditionally. The correct behavior is to check if
"ext4_share_dup_blocks" is set to "true".

Test: 1. build bonito with "BOARD_EXT4_SHARE_DUP_BLOCKS := false"
      2. check for "shared_blocks" feature of system.img
         ./out/host/linux-x86/bin/tune2fs -l \
         ./out/target/product/bonito/system.img | \
         grep -o shared_blocks

Change-Id: I8c0e5606658dd0f47b71cfc6c453a77a57da4f42
diff --git a/tools/releasetools/build_image.py b/tools/releasetools/build_image.py
index e424b6f..ed9183e 100755
--- a/tools/releasetools/build_image.py
+++ b/tools/releasetools/build_image.py
@@ -287,7 +287,7 @@
         build_command.extend(["-U", prop_dict["uuid"]])
       if "hash_seed" in prop_dict:
         build_command.extend(["-S", prop_dict["hash_seed"]])
-    if "ext4_share_dup_blocks" in prop_dict:
+    if prop_dict.get("ext4_share_dup_blocks") == "true":
       build_command.append("-c")
     if (needs_projid):
       build_command.extend(["--inode_size", "512"])