mkimage: active first partition (fat32) for bbb and others.

I activated the second partition by accident when the code was changed
to use sfdisk. Found and fixed by @pullmoll.

Close #80

mkimage.sh.in

@@ -154,8 +154,8 @@ _EOF
 *)
     sfdisk ${FILENAME} <<_EOF
 label: dos
-2048,${BOOT_FSSIZE},b
-${BOOT_SIZE},+,L,*
+2048,${BOOT_FSSIZE},b,*
+${BOOT_SIZE},+,L
 _EOF
     LOOPDEV=$(losetup --show --find --partscan $FILENAME)
     mkfs.${BOOT_FSTYPE} $_args ${LOOPDEV}p1 >/dev/null