infra/pkg-kconfig: do not rely on package's .config as a timestamp

Since linux-4.19, the kernel's build system internally touches its
.config file.

However, we currently used that file as a timestamp to detect whether
our kconfig fixups were to be (re)applied or not, which in turn is used
to decide whether we should (re)build the package or not.

But with latest kernel versions, this timestamp heuristic is now broken,
and we always rebuild the kernel on subsequent builds.

We fix that by introducing a separate timestamp file of our own, which
we know the kernel (or the kconfig-based packages, for that matters)
does not use.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>