[configs/baseline] Use relative paths for syslinux like in releng profile

Signed-off-by: Gerardo Exequiel Pozzi <vmlinuz386@yahoo.com.ar>
[configs/baseline] Use gzip (default) instead of xz

Signed-off-by: Gerardo Exequiel Pozzi <vmlinuz386@yahoo.com.ar>
[configs/baseline] Rework build.sh add run_once() like in releng profile and other misc things

Signed-off-by: Gerardo Exequiel Pozzi <vmlinuz386@yahoo.com.ar>