apm821xx: switch MR24's initramfs to multi-image method

apm821xx: switch MR24's initramfs to multi-image method

The recent change to switch to gcc 7.3 broke the image
generation code, as the kernel would no longer fit into
KERNEL_SIZE.

This patch fixes the issue by reworking the initramfs
creation and packaging, which will get rid of the
KERNEL_SIZE check in the process.

This new initramfs can be loaded through the MR24 U-boot
in the following way:

=> setenv ipaddr 192.168.1.1
=> setenv bootargs console=ttyS0,$baudrate
=> tftpboot c00000 192.168.1.2:meraki_mr24-initramfs-kernel.bin
[...]
Load address: 0xc00000
Loading: ################################################ [...]
done
    Bytes transferred = 5952544 (5ad420 hex)
    => bootm $fileaddr
    \## Booting kernel from Legacy Image at 00c00000 ...
    ...

For more information and the latest flashing guide:
please visit the OpenWrt Wiki Page for the MR24:
<https://openwrt.org/toh/meraki/mr24#flashing>

Cc: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Chris Blake <chrisrblake93@gmail.com>