classes/rm_work: rename RM_WORK_WHITELIST to RM_WORK_EXCLUDE

This really functions as a blacklist, not a whitelist, since we are
listing recipes to exclude. To avoid any possibility of confusion, since
this was a recent addition, rename the variable.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>