icecc: Use bb.utils.which instead of calling which

icecc: Use bb.utils.which instead of calling which

The icecc class often calls 'which' for determining paths. This leads
to many messages on stderr in case 'which' doesn't find the
executable. Using bb.utils.which is more appropriate here and doesn't
pollute stderr.

(From OE-Core rev: 7ed7aa38182ce8c7300a08e3aefcc65da2b524a8)

Signed-off-by: Tobias Henkel <tobias.henkel@bmw-carit.de>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>