flash: Generate hashsums for IMG package

Description

flash: Generate hashsums for IMG package

Issue(s)

https://gitlab.e.foundation/e/devices/backlog/-/issues/271

Test environment

• Make an FP4 build with the following parameters:
  • DEVICES: FP4
  • BRANCH_NAME: v1-s
  • FEATURE_BRANCH: 271devices-s-IMG_hashsums

Test procedures

FP4

Job: https://gitlab.e.foundation/e/priv/os/build/-/jobs/398289

• Ensure IMG-e*.zip.md5sum is present and valid
• Ensure IMG-e*.zip.sha256sum is present and valid

Check list

• Self review
• Test procedure explained
• Tested on fresh install
• Tested applied as an update
• License
• Internal documentation
• User documentation