libpayload: Add a Makefile for in-tree payloads

The new `Makefile.payload` can be included by the Makefiles of pay-
loads for in-tree builds. The basic idea is to use libpayload's
build results without the `make install` step, and to ensure that
incremental builds work. For instance, if libpayload's code changes,
a `make` for the payload would automatically update the libpayload
build and rebuild the payload. But if there are no code changes in
libpayload, only updated files of the payload will be re-built.

The configuration of libpayload is supposed to be automatically
generated from a `defconfig` file. If this `defconfig` changes,
libpayload and the payload will be re-built.

Change-Id: If5319f1bf0bcd09964416237c5cf7f8e59f487a2
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/47633
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

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