scripts: build-all: Add a keep-going option

Sometimes I want to build all the targets and see what errors
come out while I'm out at lunch. Currently I can't do that so
introduce a new option -k/--keep-going in the spirit of make's
keep going option to build the rest of the targets even if
another target fails. This way after I eat lunch I can go through
the logs and see what files are failing.

It was suggested that the failing targets could be summarized at
the end of the build run. So when we're building many targets
keep a list of failing targets to output at the end of the run.
This way it's easy to determine which targets need fixing without
scouring the logs or the screen buffer.

While we're here fixup the above usage message.

Change-Id: Ief30ac65eb946e055051bdfdd7ba5788e5a8e426
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
(cherry picked from commit 3fb23529a8c959042d3efb68888bc18d8fbce0ce)