Document BSDMAKE variable that was recently introduced.  Also, since RELENG_7, the way to do a cross build changed from specifying only TARGET_ARCH (which has an ambigous mapping to TARGET) to specifying only TARGET (which has a unique mapping to TARGET_ARCH).  Update the example to reflect this.
Document BSDMAKE variable that was recently introduced.  Also, since
RELENG_7, the way to do a cross build changed from specifying only
TARGET_ARCH (which has an ambigous mapping to TARGET) to specifying
only TARGET (which has a unique mapping to TARGET_ARCH).  Update the
example to reflect this.

MFC After: 1 week (the example change only)

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase