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/binutils-2.38/binutils-2.38.sh

@@ -89,7 +89,7 @@ src_configure() {
 src_compile() {
     make -C bfd headers
     for dir in libiberty bfd opcodes libctf binutils gas gprof ld; do
-        make -C $dir tooldir=${PREFIX} CFLAGS="-std=c99"
+        make "${MAKEJOBS}" -C $dir tooldir=${PREFIX} CFLAGS="-std=c99"
     done
 }