coreboot_util: restrict serial update to standard firmware

Signed-off-by: Matt DeVillier <matt.devillier@puri.sm>
3 jobs for test
Status Job ID Name Coverage
  Build
failed #12311
build

 
  Test
skipped #12313
allowed to fail
code_quality
skipped #12312
test
 
Name Stage Failure
failed
build Build There has been a timeout failure or the job got stuck. Check your timeout limits or try again
No job log