Fix checksum calculation:

Fix checksum calculation:
1.  Iterate over all partitions counted in the label, which can be more
   than the number of partitions given to mkimg(1).
2.  Start the checksum from the beginning of the label; not the beginning
   of the bootarea.

Tested with bsdlabel(8).

Approved by:	re@ (rodrigc)

git-svn-id: svn://svn.freebsd.org/base/stable/10@271843 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f