support/kconfig: add support for olddefconfig

olddefconfig is a new target available in kconfig that allows to take
an old .config file, and update it to the latest version of the code,
without being asked for questions: it automatically assumes the
default value for options whose value was not defined.

It for example allows to take a minimal defconfig, copy it as .config,
and do 'make olddefconfig' to get the full .config.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Tested-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>