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/bash-5.2.15/bash-5.2.15.sh

@@ -47,6 +47,10 @@ src_configure() {
         bash_cv_dev_fd=whacky
 }
 
+src_compile() {
+    make -j1 PREFIX="${PREFIX}"
+}
+
 src_install() {
     install -D bash "${DESTDIR}${PREFIX}/bin/bash"
     # Work around weird symlink bug