ci-build: Options to not wipe TMP/DL/SSTATE between builds
ci-build: Options to not wipe TMP/DL/SSTATE between builds

Additional environment variables allow to wipe tmp/ downloads/ and
sstate-cache/, or to keep and reuse them.  The default is (as before) to
always wipe them before building.

GENIVI/genivi-dev-platform,GENIVI/genivi-dev-platform,chbae/genivi-dev-platform,chbae/genivi-dev-platform,chbae/genivi-dev-platform,chbae/genivi-dev-platform,GENIVI/genivi-dev-platform,gunnarx/genivi-dev-platform,gunnarx/genivi-dev-platform,gunnarx/genivi-dev-platform,GENIVI/genivi-dev-platform,gunnarx/genivi-dev-platform