scripts: build-all: Allow Limit load average on builds.

Accept the -l make argument as well as -j.  Neither of these
arguments now have a default, other than what make uses, allowing
the user to specify whatever value they wish.

Signed-off-by: David Brown <davidb@quicinc.com>
(cherry picked from commit 0b2e450e1478711a0d19f15a7e66dec742468d7b)