coreboot_util: exit script if updating via Heads

Exit script immediately rather than returning to the
main menu so that the filename remains visible to the
user / makes it easier to copy to USB

Signed-off-by: Matt DeVillier <matt.devillier@puri.sm>
3 jobs for master
Status Job ID Name Coverage
  Build
canceled #12302
build

 
  Test
canceled #12304
allowed to fail
code_quality

canceled #12303
test