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/gc-8.0.4/gc-8.0.4.sh

@@ -11,6 +11,6 @@ src_configure() {
     ./configure \
         --prefix="${PREFIX}" \
         --libdir="${LIBDIR}" \
-	--disable-shared \
+        --disable-shared \
         CFLAGS='-D_GNU_SOURCE -DNO_GETCONTEXT -DSEARCH_FOR_DATA_START -DUSE_MMAP -DHAVE_DL_ITERATE_PHDR'
 }