fix CRC32 calculation for sysupgrade

* reduce image size for CRC calculation by fs_mark size

sysupgrade sometimes failed for me and I noticed that it was due
to incorrect CRC values in trx-header after performing it.
It seems that the fs_mark was completely included in the calculation
and that it was nevertheless modified by sysupgrade while appending
the jffs data.
This only occurs for the first boot after sysupgrade as the flashmap
driver recalculates the CRC to an even smaller area when it boots.

SVN-Revision: 22396