contrib: setup_build_env: remove bb layers
contrib: setup_build_env: remove bb layers

When building several images, bb layers from previous builds are left
behind.  These layers can conflict and result in the wrong build.  This
change cleans up those bb layers.

Signed-off-by: Michael West <e44a0eed3e111b03872b778357f52e3d471cf187@ettus.com>

EttusResearch/meta-ettus,EttusResearch/meta-ettus,EttusResearch/meta-ettus,EttusResearch/meta-ettus,EttusResearch/meta-ettus,EttusResearch/meta-ettus