pkg-kconfig: support alternative .config location

pkg-kconfig: support alternative .config location

Kconfig clones, such as openconf used by xvisor [1], do not look for
.config at the root of the build directory, but in a subdirectory (e.g.
build/openconf).

This patch introduces a new Makefile variable named
$(2)_KCONFIG_DOTCONFIG, which contains the path to .config relative to
the source directory of the package. It defaults to .config and can be
overridden in the package Makefile.

It also creates the full directory hierarchy where the .config file is
stored, in case it does not exist (e.g. in xvisor).

This allows the use of the kconfig-package infrastructure with packages
relying on such clones.

[1] https://github.com/xvisor/xvisor/tree/master/tools/openconf

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[Thomas: use "install -D" instead of "mkdir + cp", as suggested by
Yann.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>