apm821xx: add size check for initramfs kernel for the Meraki MR24

apm821xx: add size check for initramfs kernel for the Meraki MR24

Russell Senior reported an issue with the MR24 initramfs kernels:
> ## Booting kernel from Legacy Image at 00c10000 ...
>   Image Name:   POWERPC LEDE Linux-4.4.19
>   Created:      2016-08-31  11:57:05 UTC
>   Image Type:   PowerPC Linux Kernel Image (gzip compressed)
>   Data Size:    2155723 Bytes = 2.1 MiB
>   Load Address: 00000000
>   Entry Point:  00000000
>   Verifying Checksum ... OK
>Wrong Ramdisk Image Format
>Ramdisk image is corrupt or invalid

For the MR24, the kernel is uploaded to 0x10000. The ramdisk starts
at 0x200000. This leaves the kernel with just 0x1f0000 bytes = 1984kb.
This patch adds a size check so the image creation script will abort
instead of producing a unbootable initramfs image. A separate patch
"apm821xx: Fix initramfs image for the Meraki MR24" which fixed the
reported issue was submitted earlier.

Cc: Russell Senior <russell@personaltelco.net>
Cc: Chris Blake <chrisrblake93@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>