package/pkg-kconfig: handle KCONFIG_DEPENDENCIES with per-package directories

package/pkg-kconfig: handle KCONFIG_DEPENDENCIES with per-package directories

The pkg-kconfig infrastructure hijacks the regular chain of build
steps to insert its own step to prepare the configuration of kconfig
packages. This additional step may have dependencies of its own, such
as host-flex, host-bison or toolchain.

In the context of per-package directory support, those dependencies
must be copied to the per-package directory of the current package
prior to doing the config preparation. This commit implements this
logic by adding a call to prepare-per-package-directory at the right
spot.

Reported-by: Andreas Naumann <anaumann@ultratronik.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>