kernel.bbclass: fix do_sizecheck() comparison

The routine do_sizecheck() was historically needed by legacy
devices with limited flash memory.
The lowest extreme is probably with Zaurus collie having exactly
1024*1024 = 1048576 bytes for the kernel partition.

In the years the KERNEL_IMAGE_MAXSIZE has been converted to kilobytes
thus rounded so we have now KERNEL_IMAGE_MAXSIZE_collie = "1024".

The effect is that now the check fails because we hit curiously this
 | WARNING: This kernel zImage (size=1024(K) > 1024(K)) is too big for...
even though zImage is 1047288 bytes (kernel + kexecboot-klibc-initramfs).

Fix this case using test -gt (greater) instead of -ge (greater or equal).

Signed-off-by: Andrea Adami <andrea.adami@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 254ca956)
Signed-off-by: Steve Sakoman <steve@sakoman.com>