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