apm821xx: switch MyBook Live's recovery images to multi-file

This patch converts the MyBook Live's recovery image to utilize the
multi-image method which integrates the device-tree binary directly
into the image.

The new initramfs can be loaded through the MyBook Live's U-boot
in the following way:

=> setenv ipaddr 192.168.1.1
=> setenv serverip 192.168.1.2
=> sata init; run addtty; tftp $kernel_addr_r wd_mybooklive-initramfs.bin; bootm
   Waiting for PHY auto negotiation to complete... done
   ENET Speed is 1000 Mbps - FULL duplex connection (EMAC0)
   Using ppc_4xx_eth0 device
   TFTP from server 192.168.1.2; our IP address is 192.168.1.1
   Filename 'wd_mybooklive-initramfs.bin'.
   Load address: 0x1000000
   Loading: ################################################ [...]
   done
   [...]
   Booting kernel from Legacy Image at 01000000 ...
      Image Name:    initramfs
      Image Type:   PowerPC Linux Multi-File Image (gzip compressed)
   [...]

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

target/linux/apm821xx/image/Makefile

@@ -228,7 +228,7 @@ define Device/wd_mybooklive
   BLOCKSIZE := 1k
   DTB_SIZE := 16384
   KERNEL := kernel-bin | dtb | gzip | uImage gzip
-  KERNEL_INITRAMFS := kernel-bin | dtb | gzip | uImage gzip
+  KERNEL_INITRAMFS := kernel-bin | gzip | dtb | MuImage-initramfs gzip
   BOOT_SIZE := 8
   IMAGES := factory.img.gz kernel.dtb sysupgrade.img.gz
   DEVICE_DTB := apollo3g.dtb