The bootloader is put in place using the instructions at
u-boot is re-compiled from the 2016.03 vanilla source which fixes a problem with Alpine Linux's u-boot attempting to scan inappropriuate mmc partitions.
If you don't compile u-boot using uboot/build_bbb.sh then the original Alpine bootloader will be
used. Otherwise the compiled version will be installed on the card image.
A single ext-4 (non-journaling) partition is created on the disk image to keep the boot files