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).

MFC after:	3 days


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271448 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f