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>