Fix flags argument passed to avbtool for GSI images

* avbtool expect extra flags to be passed with the --flags argument
  but currently we're missing an 's'.
  Luckily this isn't an issue as python argparse matches
  --flag as an alias to --flags ( python... )

Test: m vbmetaimage; check vbmeta header
Change-Id: I42d833c61c9b313dc067b1e519bae36395d09824