core/Makefile: also publish initrd into /system/boot so it can be used by ubuntu

Signed-off-by: Ricardo Salveti de Araujo <ricardo.salveti@canonical.com>
Signed-off-by: Sergio Schvezov <sergio.schvezov@canonical.com>
Merge commit '0f9b612581c75aff9840b37e1b36999e7de92603' into phablet-10.1