core/Makefile: start building ubuntu-boot.img with ubuntu-touch specific initrd

Signed-off-by: Ricardo Salveti de Araujo <ricardo.salveti@canonical.com>
Signed-off-by: Sergio Schvezov <sergio.schvezov@canonical.com>
core.mk: also building libui_compat_layer

Signed-off-by: Ricardo Salveti de Araujo <ricardo.salveti@canonical.com>