mkimage.sh: fix sfdisk usage for multiple arches.

In the case where we have two partitions, boot and root, the
root partition needs to start at sector (2048 + bootfs_size_in_sectors),
not at $BOOT_FSSIZE.
Using $BOOT_FSSIZE as the start tells sfdisk to try to start the
/ partition in the middle of /boot, which it refuses to do.
Leave start blank, and sfdisk uses the first unallocated sector as
the start of /, which happens to be sector 2048 + bootfs_size_in_sectors.

I suspect this fixes #81.

mkimage.sh.in

@@ -155,7 +155,7 @@ _EOF
     sfdisk ${FILENAME} <<_EOF
 label: dos
 2048,${BOOT_FSSIZE},b,*
-${BOOT_FSSIZE},+,L
+,+,L
 _EOF
     LOOPDEV=$(losetup --show --find --partscan $FILENAME)
     mkfs.${BOOT_FSTYPE} $_args ${LOOPDEV}p1 >/dev/null