infra: introduce a kconfig-package infrastructure

There are several packages that have a configuration file managed by
kconfig: uclibc, busybox, linux and barebox. All these packages need some
make targets to handle the kconfig specificities: creating a configuration
(menuconfig, ...) and saving it back (update-config, ...)

These targets should be the same for each of these packages, but
unfortunately they are not. Especially with respect to saving back the
configuration to the original config file, there are many differences.

A previous set of patches fixed these targets for the uclibc package.
This patch extracts these targets into a common kconfig-package
infrastructure, with the goals of:
- aligning the behavior of all kconfig-based packages
- removing code duplication

In order to use this infrastructure, a package should at a minimum specify
FOO_KCONFIG_FILE and eval the kconfig-package macro. The supported
configuration editors can be set with FOO_KCONFIG_EDITORS and defaults to
menuconfig only.
Additionally, a package can specify FOO_KCONFIG_OPT for extra options to
pass to the invocation of the kconfig editors, and FOO_KCONFIG_FIXUP_CMDS
for a list of shell commands used to fixup the .config file after a
configuration has been created/edited.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
[yann.morin.1998@free.fr: add missing 4th argument when calling to
 inner-kconfig-package (namely, 'target']
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>