Some partitions are sparse (DATAFS, FACTORYFS), but the largest
partition, UMS, needs to be non-sparse. DATAFS partition also expects
some special images with a magic header (or something), simply trying
to flash a sparse or non-sparse image to it gives an error from device
bootloader:
```
[ ... ]
erase completed(from 73728 to 90111).
.
mmc_pwrite: partition[6] written successfully.
...set_nps_update_start: set nps start flag successfully.
process_packet: id=102, data=0
process_packet: id=102, data=2
process_packet: id=102, data=3
pda completed: id=11, size=31457280, final=0
Invalid Magic Code! 0x 0
[ ... ]
```
Therefore set UMS as default rootfs partition and create non-sparse
images.
Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/8877>
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