linux-defconfig: explicit cross compile flag usage

Current linux config file generation does not add the cross compilation
flag, leading potentially to compilation errors. Not sure how a proper config
file is generated with current implementation but issues are observerd
specially when buildling the linux image with a single thread.

This commit ensures that both ARCH and CROSS_COMPILE flags are set when
using the merge_config script.

Signed-off-by: Ibai Erkiaga <ibai.erkiaga-elorza@xilinx.com>
Reviewed-by: Jerome Forissier <jerome@forissier.org>
diff --git a/common.mk b/common.mk
index cd62a1e..f72ea86 100644
--- a/common.mk
+++ b/common.mk
@@ -376,6 +376,7 @@
 $(LINUX_PATH)/.config: $(LINUX_DEFCONFIG_COMMON_FILES)
 	cd $(LINUX_PATH) && \
 		ARCH=$(LINUX_DEFCONFIG_COMMON_ARCH) \
+		CROSS_COMPILE=$(CROSS_COMPILE_NS_KERNEL) \
 		scripts/kconfig/merge_config.sh $(LINUX_DEFCONFIG_COMMON_FILES) \
 			$(LINUX_DEFCONFIG_BENCH)