Use common rule for linux/.config and per-platform config fragments

- Kernel config fragments that were used by the QEMU makefile are
merged into kconfigs/qemu.conf. Now, each platform has its own file
in kconfigs/
- All platforms: CONFIG_DMA_SHARED_BUFFER cannot be enabled directly
because it is  a not a visible symbol (has no prompt in Kconfig).
CONFIG_DRM is set instead which depends on DMA.
- kconfigs/hikey.conf: keep only what is necessary

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Tested-by: Jerome Forissier <jerome.forissier@linaro.org> (FVP, HiKey)
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Tested-by: Joakim Bech <joakim.bech@linaro.org> (QEMU, MTK8173)
diff --git a/qemu.mk b/qemu.mk
index eb3dace..a7a117f 100644
--- a/qemu.mk
+++ b/qemu.mk
@@ -79,13 +79,10 @@
 ################################################################################
 # Linux kernel
 ################################################################################
-KCONFIGS := kconfig_preempt.conf kconfig_no_jffs2.conf
-$(LINUX_PATH)/.config: $(KCONFIGS)
-	# Temporary fix until we have the driver integrated in the kernel
-	sed -i '/config ARM$$/a select DMA_SHARED_BUFFER' $(LINUX_PATH)/arch/arm/Kconfig;
-	cd $(LINUX_PATH) && ARCH=arm scripts/kconfig/merge_config.sh \
-		arch/arm/configs/vexpress_defconfig \
-		$(patsubst %,$(CURDIR)/%,$(KCONFIGS))
+LINUX_DEFCONFIG_COMMON_ARCH := arm
+LINUX_DEFCONFIG_COMMON_FILES := \
+		$(LINUX_PATH)/arch/arm/configs/vexpress_defconfig \
+		$(CURDIR)/kconfigs/qemu.conf
 
 linux-defconfig: $(LINUX_PATH)/.config