[makefile] Allow buit-in compilation

[maximeborges]

Hello,
I haven't done a lot a kernel work, so I'm not sure this is the cleanest way to do that, but here you go.
The README modification should be self-explanatory, but in case, the goal is to be able to build this module in-tree

[ikwzm]

Thank you for the PR.

However, I don't understand the significance of changing the Makefile. Isn't the drivers/of/ Makefile used when building into drivers/of ?

[maximeborges]

Since it's used here as a submodule and not adding the source directly to the kernel source, it sounds better to me to include the folder with obj-$(CONFIG_OF_DTBOCFG) += dtbocfg/ instead of the source file directly in drivers/of/Makefile.

Then the modification in the Makefile of dtbocfg makes it compile by default as a module if built out-of-tree, or in-tree if CONFIG_OF_DTBOCFG set to y.

Not sure about the good practice for such things tho, never done that before.