We need to be able to work with ISO images inside builder container.
This change makes the stx-builder privileged so we can mount a
filesystem inside it and we add mkisofs, isomd5sum and syslinux-utils
packages to be installed when building the image.
Test plan:
PASS: Build stx-builder image.
PASS: Run ./stx-init-env --rebuild with success.
Story: 2010676
Task: 50189
Change-Id: I44b0d99965befd9e78934ff3dbcbe4bdf02514af
Signed-off-by: Dostoievski Batista <dostoievski.albinobatista@windriver.com>