scripts: fix regression on reuse of shell environment variables

instead of editing local.conf to defines DL_DIR and SSTATE_DIR,
the shell environment variables can be propagated in poky/oe build env.

BB_ENV_EXTRAWHITE is defined in "poky/scripts/oe-buildenv-internal"
Thus, variable append step should occurs _after_ setup poky/oe.

Change-Id: I5d4901435812d2d82950588a89b8ec67f2b01ea8
Signed-off-by: Yannick Gicquel <yannick.gicquel@iot.bzh>