scripts: build-all: add jobs option

It's annoying that the number of make jobs is hardcoded at 6.
Allow users to define the number of jobs they want with a new
option in the spirit of '-j' from make.

Change-Id: I27881eb604e27a995d5d70bdeabacd6f690aa5b0
Signed-off-by: Stephen Boyd <sboyd@quicinc.com>
(cherry picked from commit 604baf789feb66fee36ea68487d21ea94d8fb374)