infra/pkg-kconfig: incorporate oldconfig call to avoid endless rebuilds

The configure step of the busybox package performs 'make oldconfig', which
causes the .config file to be updated. Thus, the .config file is more recent
than our stamp file .stamp_kconfig_fixup_done. On a subsequent build, our
dependency rules would kick in, and run the config fixup again, thus
kicking in the package's configure, build and install steps yet once
more, that, ad infinitum.

One solution is to modify kconfig-package to introduce an explicit touch of
the .kconfig_fixup_config_done stamp file, as post-configure hook.

Another solution, implemented by this patch, is to move the oldconfig call
from the package's .mk file to the kconfig-package infrastructure and make
sure it is done as part of the fixup commands. This way, the stamp file will
only be touched once, after the full fixup (including oldconfig) and no
endless rebuilds will occur.

Reported-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Suggested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>