fix manifest generation

When FFS_BUILD_DATE is not set when make manifest is executed, an manifest
without any images will be generated.
3 jobs for v1.9+2020-06-06
latest
Status Name Job ID Coverage
  Generate Date
failed generate_date #2298

 
  Update
skipped update #2299
 
  Package
skipped package #2300
 
Name Stage Failure
failed
generate_date Generate Date There has been a timeout failure or the job got stuck. Check your timeout limits or try again
No job log