zephyr: Automatically derive ARCH.

Currently to compile for anything that except ARCH=x86 we have to
provide ARCH via the environment or make arguments. We can do better
than that!
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>

zephyr/Makefile

@@ -9,13 +9,18 @@
 # Zephyr build system.
 #
 
-ARCH ?= x86
 BOARD ?= qemu_x86
 # Zephyr 1.5.0
 #OUTDIR_PREFIX =
 # Zephyr 1.6.0
 OUTDIR_PREFIX = $(BOARD)
 
+Z_DOTCONFIG = outdir/$(OUTDIR_PREFIX)/.config
+DQUOTE = "
+# "
+include $(Z_DOTCONFIG)
+override ARCH = $(subst $(DQUOTE),,$(CONFIG_ARCH))
+
 # Zephyr toolchain config is 2-pass, so included twice
 include $(ZEPHYR_BASE)/scripts/Makefile.toolchain.$(ZEPHYR_GCC_VARIANT)
 include $(ZEPHYR_BASE)/scripts/Makefile.toolchain.$(ZEPHYR_GCC_VARIANT)
@@ -50,6 +55,9 @@ CFLAGS = -std=gnu99 $(TOOLCHAIN_CFLAGS) $(INC) $(CFLAGS_MOD) $(COPT) $(CFLAGS_TA
 
 include ../py/mkrules.mk
 
+$(Z_DOTCONFIG):
+	$(MAKE) -f Makefile.zephyr BOARD=$(BOARD) initconfig
+
 all: $(LIBMICROPYTHON)
 	$(MAKE) -f Makefile.zephyr BOARD=$(BOARD)