zephyr/Makefile: syscall_macros.h generation was moved from CMake to make.

Required for #include <zephyr.h> to work.

ports/zephyr/Makefile

@@ -105,3 +105,4 @@ outdir/$(BOARD)/Makefile: $(CONF_FILE)
 
 $(Z_EXPORTS): outdir/$(BOARD)/Makefile
 	make --no-print-directory -C outdir/$(BOARD) outputexports CMAKE_COMMAND=: >$@
+	make -C outdir/$(BOARD) syscall_macros_h_target