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>

script/update/trees

@@ -172,7 +172,7 @@ check_cross_compiler()
 		[ -n "${xlang}" ] && export BUILD_LANGUAGES="$xlang"
 
 		[ -d "${cbdir}/util/crossgcc/xgcc/${_xarch}/" ] && continue
-		x_ make -C "$cbdir" crossgcc-${_xarch%-*} CPUS=$(nproc)
+		x_ make -C "$cbdir" crossgcc-${_xarch%-*} CPUS=$threads
 	done
 }
 
@@ -219,7 +219,7 @@ run_make_command()
 	[ "$project" = "coreboot" ] && [ -z "$mode" ] && x_ \
 		printf "%s\n" "${version%%-*}" > "$codedir/.coreboot-version"
 
-	make $mode -j$(nproc) $makeargs -C "$codedir" || \
+	make $mode -j$threads $makeargs -C "$codedir" || \
 	    $err "run_make $codedir: !make $mode"
 
 	[ "$mode" != "clean" ] && return 0