Introduce parallelism

By using --cores argument to rootfs.py, JOBS= is set in the
live-bootstrap environment, and -j${JOBS} is used on builds. This speeds
larger packages up significantly.

A fair number of packages do not build properly with parallelism. Most
of these, at least for now, are disabled with -j1.

sysc/init

@@ -21,6 +21,8 @@ export SOURCE_DATE_EPOCH=0
 # shellcheck source=sysa/helpers.sh
 . "${SOURCES}/helpers.sh"
 
+MAKEJOBS="-j${JOBS}"
+
 echo
 echo "Installing packages into sysc"
 
@@ -75,6 +77,7 @@ SOURCES=${SOURCES}
 DESTDIR=${DESTDIR}
 DISTFILES=${DISTFILES}
 SRCDIR=${SRCDIR}
+MAKEJOBS=${MAKEJOBS}
 EOF
 
 exec ./run.sh