icecc: don't create unnecessary 'ice' dirs in sysroots when disabled

* parsing ICE_PATH="${@icc_path(bb, d)}" causes "ice" directories
  to be created in every sysroot, that could be a bit confusing for
  people who inherit icecc.bbclass, but disabled it
* shorten ICECC_VERSION="${@icc_version(bb, d)}" path a bit
  by returning sooner when disabled
* remove ICECC_PATH and ICECC_ENV_EXEC from signatures, we assume that
  using icecc doesn't influence the output, so it shouldn't matter when
  user supplies own version of icecc or env script
* always compare ICECC_DISABLED with "1", boolean typed_value isn't used
  because documentation already mentions using empty value to keep icecc
  enabled and that's not valid boolean value when oe.data.typed_value is
  used:
  ERROR: ICECC_DISABLED: Invalid boolean value ''

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>