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>

 # Options specific to crosstool-NG acting as a backend

 config IS_A_BACKEND

     string

     option env="CT_IS_A_BACKEND"

 config BACKEND

     bool

     default y if IS_A_BACKEND = "y" || IS_A_BACKEND = "Y"

 config BACKEND_ARCH

     string

     option env="CT_BACKEND_ARCH"

 config BACKEND_KERNEL

     string

     option env="CT_BACKEND_KERNEL"

 config BACKEND_LIBC

     string

     option env="CT_BACKEND_LIBC"