allow users to specify number of build threads
    
    lbmk otherwise uses nproc to set the number of build threads,
    in these places:
    
    * generic make commands in script/update/trees
    * crossgcc make command in script/update/trees
    
    the -T0 option is also used in script/update/release, when running
    tar.
    
    with this change, you can do:
    
    export LBMK_THREADS=x
    
    where x is the number of threads. when you then run
    lbmk, your chosen number of threads will override
    the default. this may be useful on a host that does
    not have a lot of memory.
    
    Signed-off-by: Leah Rowe <leah@libreboot.org>