Implement grub boot script

Acceptance criteria:

• There must a boot script for grub that can:
  • Select correct boot partition based on configuration in mender_boot_part.conf
  • Keep a boot counter in mender_boot_counter.conf that will roll back if if exceeded
  • It will not roll back if mender_boot_counter.conf doesn't exist
• For atomicity:
  • All configuration files that grub writes to should be stored on the data partition
  • All writes to files should use atomic rename over existing file
• Script does not need to be auto-included in the build (separate task)

The names and layout above are not 100% final, they can be adjusted if need arises during implementation.