icecc.bbclass: exclude icecc variables from signature

:Release Notes:
Exclude ICECC_PATH and ICECC_ENV_EXEC variables from signature
calculation of package shared state.

:Detailed Notes:

:Testing Performed:
Generated local shared state, which I successfully reused to build
webos image on a different machine.  Signatures change due to above
variables were not present

:QA Notes:

:Issues Addressed:
[ES-591] Enabling ICECC changes shared state

Open-webOS-DCO-1.0-Signed-off-by: Iyad Qumei <iyad.qumei@lge.com>

Change-Id: I8e85d93e0047c1314d084512bd0a112f3920e532
Reviewed-on: https://g2g.palm.com/5034
Reviewed-by: DCO Verification
Reviewed-by: Iyad Qumei <iyad.qumei@lge.com>
Tested-by: Iyad Qumei <iyad.qumei@lge.com>
Reviewed-by: Susan Montooth <susan.montooth@lge.com>