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.

sysa/flex-2.5.11/flex-2.5.11.sh

@@ -11,6 +11,10 @@ src_prepare() {
     rm parse.c parse.h scan.c skel.c
 }
 
+src_compile() {
+    make -j1 PREFIX="${PREFIX}"
+}
+
 src_install() {
     if test -d /lex; then
         # Remove lex, later make install will symlink lex to flex