Add feature for enabling the YP ptest capabilities

Add feature for enabling the YP ptest capabilities

This change adds a feature 'agl-ptest' that can be used to enable the addition
of the 'ptest' packages to the final image.
See the YP documentation about ptest:
- https://wiki.yoctoproject.org/wiki/Ptest
- http://www.yoctoproject.org/docs/latest/dev-manual/dev-manual.html#testing-packages-with-ptest

TLDR: enable feature +
      build image +
      boot image +
      login +
      call 'ptest-runner' +
      go get some coffee, go for a walk or run it just overnight

Bug-AGL: SPEC-911

Change-Id: Ifbe7b4144a9e1071295da58e26e8a8bd2260cd88
Signed-off-by: Jan-Simon Möller <jsmoeller@linuxfoundation.org>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/11327
Reviewed-by: Thomas Rini <trini@konsulko.com>
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-boot-test: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>