rather than adding '-s' to make's command line in the makewrapper,
use the .SILENT target based on MAKEVERBOSE's value.
This means that you can do:
./build.sh -N 1 makewrapper
$TOOLDIR/bin/nbmake-$MACHINE
# runs at MAKEVERBOSE==1, with command lines suppressed
$TOOLDIR/bin/nbmake-$MACHINE MAKEVERBOSE=2
# runs at MAKEVERBOSE==2 for this invocation, with command lines shown