libpayload: Rename Config.in -> Kconfig

libpayload: Rename Config.in -> Kconfig

libpayload is the only Kconfig based project under
the coreboot umbrella that is using Config.in as its
name for Kconfig config files. Rename that to Kconfig
as on the other projects for consistency.

Change-Id: I1c69ec13582d88409384b492484535dcc5e1ad20
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-on: http://review.coreboot.org/10520
Tested-by: build bot (Jenkins)
Reviewed-by: Marc Jones <marc.jones@se-eng.com>