environment variables to override the sector number, and the
bytes-offset in the sector, where disklabel(8) writes a disklabel.
If disklabel(8) was compiled as a host tool, then the defaults
still come from the port's LABELOFFSET and LABELSECTOR #defines.
Otherwise, the defaults still come from the sysctls, kern.labelsector
and kern.labeloffset.
This change helps me create a bootable CompactFlash image with both
a Master Boot Record (MBR) and a BSD disklabel for the RouterBOARD
153. The RB 153 is an evbmips board with a CompactFlash slot. It
searches the MBR on a CompactFlash card for a partition containing
an ELF kernel. When disklabel(8) is built for evbmips, it will
ordinarily overwrite an MBR with the BSD disklabel.
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