multilib.bbclass: fix qa warning of kernel-devicetree

multilib.bbclass: fix qa warning of kernel-devicetree

When kernel-devicetree is in RRECOMMENDS such as via variable
MACHINE_EXTRA_RRECOMMENDS for some bsp, it shows QA warning of multilib:

| WARNING: lib32-packagegroup-base-1.0-r83 do_package: QA Issue:
| lib32-packagegroup-base package lib32-packagegroup-machine-base
| - suspicious values 'kernel-devicetree' in RRECOMMENDS [multilib]

Add kernel-devicetree to exceptions to fix the QA issue. Because there
are already 3 kernel related criteria, simplify them by judging package
names whether start with 'kernel-'. And also refactor to remove
duplicate 'not'.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>