Instruct xbps-install to do unpack only, so packages can be properly
configured by xbps-reconfigure call.
Before, xbps-install was sometimes marking packages as configured,
thus making xbps-reconfigure call useless.
Create boot partition starting at 32768 sectors on pinebookpro
platforms. That leaves enough unallocated space to install u-boot
without overlapping with any partition.