Mimic commit 36b59c001c1643217449646b371df46d2cb11b91 in
diskimage-builder, by adopting the usage of $DIB_DEBUG_TRACE to check
whether enable tracing in scripts.
Unlike with the diskimage-builder commit, the default is to not enable
tracing even in the few scripts that used to unconditionally "set -x".
Enabling tracing can be done by either:
- passing -x to disk-image-create
- exporting DIB_DEBUG_TRACE=N, with N=0/1
Change-Id: I56ccd6753df31f7ddda641640cdb1985b2d9e856
Partial-Bug: #1435306
55a21cd746