Ok I made it this far:

/home/user # umount /dev/mmcblk1p1
/home/user # sfdisk -uM /dev/mmcblk1 <<EOF
> <140100>,C
> L
>
> EOF
Checking that no-one is using this disk right now ...
OK

Disk /dev/mmcblk1: 490432 cylinders, 4 heads, 16 sectors/track
Old situation:
Units = mebibytes of 1048576 bytes, blocks of 1024 bytes, counting from 0

Device Boot Start End MiB #blocks Id System
/dev/mmcblk1p1 0+ 15325 15326- 15693816 b W95 FAT32
/dev/mmcblk1p2 0 - 0 0 0 Empty
/dev/mmcblk1p3 0 - 0 0 0 Empty
/dev/mmcblk1p4 0 - 0 0 0 Empty
trailing junk after number

sfdisk: bad input
/home/user #
/home/user #

did i put the size to high?