jjb/common: remove hwtest.enable files

jjb/common: remove hwtest.enable files

releng-scripts now supports LAVA job generation for all boards.

Remove the check in run-test-prepare.sh on whether or not to create LAVA
test jobs. Replace this test by several exports using releng-scripts
job-prereq tool. These exports will be used to rsync the files to the CI
upload server.

Transition all platforms in run-test-short.sh to use releng-scripts.
Transition all platforms in run-test-short-release.sh to use releng scripts.

JIRA: SPEC-1144

Change-Id: I558563704bbe9bf0bf554219f4f7ca8e086649bb
Signed-off-by: Loys Ollivier <lollivier@baylibre.com>