Makefile: Include $(top) in DOTCONFIG definition to allow override

Including $(top) in the DOTCONFIG definition allows getting rid of the
$(top) prefix in payloads, which in turns allows providing a full path
for DOTCONFIG via the command line.

Change-Id: I7546a12cf4a2a146e32fef81121f45f83ba67ac8
Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Reviewed-on: https://review.coreboot.org/15826
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

Makefile

@@ -65,7 +65,7 @@ TOPLEVEL := .
 CONFIG_SHELL := sh
 KBUILD_DEFCONFIG := configs/defconfig
 UNAME_RELEASE := $(shell uname -r)
-DOTCONFIG ?= .config
+DOTCONFIG ?= $(top)/.config
 KCONFIG_CONFIG = $(DOTCONFIG)
 export KCONFIG_CONFIG
 HAVE_DOTCONFIG := $(wildcard $(DOTCONFIG))