buildscript: count available cpus with nproc

Grepping `/proc/cpuinfo` does not yield the correct number of available
cpus when running in a docker container or setting the number of
available cpus with taskset.

```
$ taskset 1 grep -c processor /proc/cpuinfo
8
$ taskset 1 nproc
1
```

This will prevent using too many build jobs on environments where the
number of available cpus is reduced.

`nproc` is part of `coreutils`.

Signed-off-by: Johannes Kimmel <fff@bareminimum.eu>

buildscript

@@ -230,7 +230,7 @@ build() {
 
     opath=$(pwd)
     cd "$builddir"
-    cpus=$(grep -c processor /proc/cpuinfo)
+    cpus=$(nproc)
 
     case "$1" in
         "debug")