[PATCH v3] Add make feature support for SoCFPGA Handoff

[Sune Brian]

> Because ".config" isn't valid, you need to have $(objdir)/.config be
> checked, and in turn that has to depend on the appropriate target (we
> have some stage that says that we are configured, I forget which).

```
make clean && make mrproper

make
***
*** Configuration file ".config" not found!
***
*** Please run some configurator (e.g. "make oldconfig" or
*** "make menuconfig" or "make xconfig").
***
make: *** No rule to make target '.config', needed by
'include/config/auto.conf'.  Stop.
```

Still not getting this, sorry a bit.

> > > spin with "make O=/tmp/somedir". It also looks like it's going to impact
> > > every board, and either fail or add some build time? Make sure both say
> > > "sandbox" and "qemu_arm" both build at least, if not a full CI run.
> >
> > For build time -> There could be 4 line of code to check.
>
> I do forget off-hand which methods of doing "something" in a Makefile
> are cheap and which result in firing off a subshell. But for a handy
> feature for one SoC I want to make sure we don't slow down say the world
> build of all 1400 configurations. For a maybe useful example, see how
> the "checkgcc6" target is handled, and maybe we can move all of this to
> live under arch/arm/mach-socfpga/config.mk and so only get invoked at
> all there.

If config.mk can also do this job why not just move it.

Brian