use getopts for the options
update buildrump.sh (sets RUMPRUN=yes in mk.conf)
Allow -s to buildnb.sh to specify alternative rumpsrc source tree
Need to build weakasm.map with RUMPRUN=no

This is a bit of a FIXXXME, since mk.conf, which defines RUMPRUN=yes,
doesn't get included except by sheer accident.
need __sigaction_sigtramp stub with latest sources
add minherit stub, required by arc4random in latest libc
set $MAKE before using it