[rbp] Update create_sdcard script so that it creates a 256MiB system partition

In addition to increasing the partition sizes, I've used consistent MiB units when creating both partitions - I think it is confusing to use cyclinders when creating the FAT partition, then switch to MiB while creating the ext4 partition.

Before (partitions created with existing create_sdcard):
```
Model: UDISK PDU09_8G AAI2.0 (scsi)
Disk /dev/sdd: 7702MiB
Sector size (logical/physical): 512B/512B
Partition Table: msdos

Number  Start    End      Size     Type     File system  Flags
 1      1.00MiB  126MiB   125MiB   primary  fat16        boot, lba
 2      132MiB   7686MiB  7554MiB  primary  ext4
```

After (partitions created with updated create_sdcard):
```
Model: UDISK PDU09_8G AAI2.0 (scsi)
Disk /dev/sdd: 7702MiB
Sector size (logical/physical): 512B/512B
Partition Table: msdos

Number  Start    End      Size     Type     File system  Flags
 1      0.00MiB  256MiB   256MiB   primary  fat16        boot, lba
 2      260MiB   7686MiB  7426MiB  primary  ext4
```