core-image-tiny-initramfs: Enable running poky-tiny on qemux86-64

Building poky-tiny for x86-64 seems fine, but when executing qemu
it complains during boot time about not being able to execute init:

[    5.409730] Failed to execute /init (error -8)

And then it drops to a login prompt (which it should't do on tiny)

This is supposed to be complaining about init's format, it only happens
on x86-64 architectures so perhaps is a 32/64 bit issue, but since
core-image-tiny-initramfs does not actually provide a traditional init,
the script is simply meant to drop to shell, we can workaround the issue
by specifying the kernel to run the init script via busybox's sh,
dropping to shell correctly on x86-64 leaving x86 unaffected.

(From OE-Core rev: f15de8cf177f8f736c180c1dcaa942a9931865d4)

Signed-off-by: Alejandro Hernandez <alejandro.hernandez@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-core/images/core-image-tiny-initramfs.bb

@@ -39,3 +39,5 @@ python tinyinitrd () {
 }
 
 IMAGE_PREPROCESS_COMMAND += "tinyinitrd;"
+
+QB_KERNEL_CMDLINE_APPEND += "debugshell=3 init=/bin/busybox sh init"