writing u-boot to a partition instead of the raw disk

UniversalSuperBox in the pine64 chat mentioned that instead of dd'ing the u-boot SPL to the correct offset of the raw disk it's also possible to create an partition at the correct offset instead and then dd'ing the SPL to that partition without offset.

# current situation
$ dd if=u-boot.img of=/dev/mmcblk0 bs=1k seek=8

# new situation
$ dd if=u-boot.img of=/dev/mmcblk0p1

This won't change a lot for current devices, but at least rockchip socs want the u-boot spl at an offset that clashes with the boot partition of postmarketOS.

if pmbootstrap creates a SPL partition then it could also correctly offset the pmos_Boot partition so it won't clash anymore.

One issue with this is that pmos currently uses the MBR partition format which is limited to 4 partitions (unless an extended partition is used which can't be used for this trick) so for some more complicated devices with multiple firmware stages we'll need to support GPT partitioning.