apm821xx: switch MX60(W)'s recovery images to multi-image method

apm821xx: switch MX60(W)'s recovery images to multi-image method

In the past, the MX60(W)'s recovery images always had problems
with the size restriction and never really worked without manual
intervention. But starting with 4.19, the MX60(W)'s kernel image
outgrew the allocated space for sysupgrade images as well. Hence
This patch reworks the initramfs, which allows the device to ease
up on the impossible tight kernel size requirements for the
sysupgrade creation and packaging. And as a result, the now
orphaned special ramdisk setup is removed in the process.

This new initramfs can be loaded through the MX60(W) U-boot
in the following way:

=> setenv bootargs console=ttyS0,$baudrate
=> tftpboot $meraki_loadaddr meraki_mx60-initramfs-kernel.bin
   [...]
   Load address: 0x800000
    Loading: ################################################ [...]
    done
   [...]

=> bootm $fileaddr
   \## Booting kernel from Legacy Image at 00800000 ...
    ...

Updated Flashing instructions for new installations which integrates
the new recovery method. Users of existing installations that only
want to sysupgrade don't need to update their existing u-boot env.

=> setenv owrt_load1 ubi read \${meraki_loadaddr} kernel
=> setenv owrt_load2 ubi read \${meraki_loadaddr} recovery
=> setenv lede_bootkernel bootm \${meraki_loadaddr_kernel} - \${meraki_loadaddr_fdt}
=> setenv owrt_bootkernel bootm \${meraki_loadaddr}
=> setenv owrt_bootargs setenv bootargs console=ttyS0,\${baudrate} rootfstype=squashfs mtdoops.mtddev=oops
=> setenv owrt_boot run meraki_ubi owrt_bootargs\; run owrt_load1 meraki_checkpart lede_bootkernel\; run owrt_load2 owrt_bootkernel
=> setenv bootcmd run owrt_boot
=> saveenv

For more information and the latest flashing guide:
please visit the OpenWrt Wiki Page for the MR60:
<https://openwrt.org/toh/meraki/mx60#flashing>

Signed-off-by: Chris Blake <chrisrblake93@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>