mpc85xx: Fix Aerohive HiveAP-330 initramfs image

At some point our initramfs image grew over 6MB, which is
causing an issue when uncompressing in the stock bootloader:

=> bootm 0x5000000 - 0x1000000;
   Image Name:   Linux-4.19.24
   Created:      2019-02-23   1:58:20 UTC
   Image Type:   PowerPC Linux Kernel Image (gzip compressed)
   Data Size:    6752470 Bytes =  6.4 MB
   Load Address: 00000000
   Entry Point:  00000000
   Verifying Checksum ... OK
   Booting using the fdt blob at 0x1000000
   Uncompressing Kernel Image ... Error: inflate() returned -5
GUNZIP: uncompress, out-of-mem or overwrite error - must RESET
   board to recover
   Loading Device Tree to 00ffa000, end 00fffc78 ... OK

To get around this, we need to move to an uncompressed image
for the initramfs image. While this makes a larger image, it
is thankfully bootable so people can then convert their
devices to run OpenWRT. It's worth noting the non-initramfs
image is under 3M, so it will be ages before we have any issues
with the flashed kernel.

Signed-off-by: Chris Blake <chrisrblake93@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
[keep commit message at less than 75 characters per line]

target/linux/mpc85xx/image/Makefile

@@ -11,6 +11,10 @@ define rootfs_align
 $(patsubst %-256k,0x40000,$(patsubst %-128k,0x20000,$(patsubst %-64k,0x10000,$(patsubst squashfs%,0x4,$(patsubst root.%,%,$(1))))))
 endef
 
+define Build/copy-file
+	cat "$(1)" > "$@"
+endef
+
 # combine kernel and rootfs into one image
 # mktplinkfw <type> <optional extra arguments to mktplinkfw binary>
 # <type> is "sysupgrade" or "factory"
@@ -80,6 +84,7 @@ define Device/hiveap-330
   BLOCKSIZE := 128k
   KERNEL_NAME := zImage
   KERNEL_SIZE := 8m
+  KERNEL_INITRAMFS := copy-file $(KDIR)/vmlinux-initramfs | uImage none
   SUPPORTED_DEVICES := aerohive,hiveap-330
   IMAGES := fdt.bin sysupgrade.bin
   IMAGE/fdt.bin := append-dtb