[PATCH] Makefile: perform sanity checks on payload during build

[Tim Hutt]

>
> I would suggest leaving the recipe for fw_payload.o alone, and using a separate,
> possibly phony, target for this check. It's surprising to have the recipe for
> assembling this one file defined in a separate file.

I think the check then needs to write to a file... something like this?

$(platform_build_dir)/firmware/fw_payload.o: $(FW_FDT_PATH)
$(platform_build_dir)/firmware/fw_payload.o: $(FW_PAYLOAD_PATH_FINAL)
$(platform_build_dir)/firmware/fw_payload.o:
$(platform_build_dir)/firmware/payload_check.stamp
$(platform_build_dir)/firmware/payload_check.stamp: $(FW_PAYLOAD_PATH_FINAL)
    @scripts/check-payload.sh $<
    @touch $@

> This can be done portably with od(1):

Ah cool, I didn't know about `od` (and googling everyone uses `xxi`
and hacky sed pipelines), thanks!

> No need for an explicit "exit 0" at the end.

Actually there is because the exit code will be the value from the
last command, which is false with the way I've written the script. Try
this script:

#!/bin/bash
false && {
  echo "Failed"
  exit 1
}

It will not print "Failed" and it will also exit with a failure exit
code. I have change the script so it's like this:

true || { ... fail ... }

But maybe it's worth keeping the explicit exit 0 just because shell is
crazy like this?