[OE-core] kernel.bbclass do_sizecheck behaviour changes

[Mike Crowe]

I discovered today that our kernel size check has been ineffective since we
took 7384d2831c713ac5999aca83c312154dc15cec56 from 2014 which changed the
units for KERNEL_IMAGE_MAXSIZE from bytes to kilobytes.

However whilst fixing that, I also noticed that
849b67b2e4820564b5e5c9bd4bb293c44351c5f3 in 2016 changed the consequences
of exceeding the size from being fatal to merely a warning.

This second change in behaviour wasn't described in the commit message, so
I'm not sure whether it was intentional. I can believe that when generating
multiple kernel image types it may not be essential that they all fit, but
I don't really know what the use case for the feature is.

I could fix this by adding to kernel.bbclass something like:

 KERNEL_IMAGE_MAXSIZE_CONSEQUENCE ?= "warn"

and then using it when delivering the bad news so that recipes can override
it if they wish.

Alternatively, I could treat the error as being fatal if none of the kernel
images fit within the required size. This would degenerate to the old
behaviour automatically if KERNEL_IMAGETYPES contains only one entry.

Does one of these solutions appeal, or is there another even better
solution?

Thanks.

Mike.