rm_work.bbclass: inhibit rm_work per recipe

rm_work.bbclass: inhibit rm_work per recipe

Use RM_WORK_WHITELIST to inhibit rm_work per recipe. In this way,
one can use rm_work for the most of the recipes but still keep the
work area for the recipe(s) one is working on.

As an example, the following settings in local.conf will inhibit
rm_work for icu-native, icu and busybox.
    INHERIT += "rm_work"
    RM_WORK_WHITELIST += "icu-native icu busybox"

If we comment out the RM_WORK_WHITELIST line and do a rebuild, the
working area of these recipes will be cleaned up.

[YOCTO #3675]

(From OE-Core rev: 6c930c3c06f2e698540626c87bd7f7f571df35ef)

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>