kernel.bbclass: update KERNEL_IMAGE_MAXSIZE

kernel.bbclass: update KERNEL_IMAGE_MAXSIZE

* Make KERNEL_IMAGE_MAXSIZE and IMAGE_ROOTFS_SIZE have the same algorithm:
  - Use Kbytes as the unit since we use this in other codes.
  - Use "du" rather than "ls" to figure out the size since we use this in
    image.py.

[YOCTO #2610]

(From OE-Core rev: 7384d2831c713ac5999aca83c312154dc15cec56)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>