config: add a 'auto' value for the number of parallel jobs

When CT_PARALLEL_JOBS is -1, set the number of parallel jobs to the
number of online CPUs + 1. Update documentation to match.

I find this useful when building in the cloud. You can use the same
.config file and have the build adapt to the number of processors
available. Limited testing shows that NCPUS+1 is faster than NCPUS+0
or NCPUS+2.

Signed-off-by: Michael Hope <michael.hope@linaro.org>

 # This file adds functions to build the C library for bare metal (IE. none)

 # Copyright 2008 Yann E. MORIN

 # Licensed under the GPL v2. See COPYING in the root of this package

 do_libc_get() {

     :

 }

 do_libc_extract() {

     :

 }

 do_libc_check_config() {

     :

 }

 do_libc_start_files() {

     :

 }

 do_libc() {

     :

 }

 do_libc_finish() {

     :

 }