apm821xx: replace recovery image for the MBL with initramfs

apm821xx: replace recovery image for the MBL with initramfs

The patch "images: bump default rootfs size to 256 MB"
a1f83bad606411a561e8e60110c71232b1a28aa2 caused a crash
during boot for the recovery images. This is because
both variants of the MyBook Live only have 256MB of RAM
and for the recovery option, the ext4 rootfs was simply
stored in the RAMDISK.

This patch replaces recovery image for the MBL with an
initramfs kernel.

In order to boot the initramfs (for recovery or development):

0. copy the initramfs and device tree into tftp's server directory
   # cp *-initramfs-kernel.bin to /tftp-server/mbl.bin
   # cp *-ext4-kernel.dtb to /tftp-server/fdt.bin

1. Connect the MyBook Live (Duo) serial port.
   (Warning! Use a 3.3v level shifter).

2. Hit Enter during u-boot and insert these three lines:
   # setenv serverip 192.168.1.254; setenv ipaddr 192.168.1.1;
   # tftp ${kernel_addr_r} mbl.bin; tftp ${fdt_addr_r} fdt.bin
   # run addtty addmisc; bootm ${kernel_addr_r} - ${fdt_addr_r}

   Where 192.168.1.254 is your TFTP server.

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