--parallel improvements (#13800)

* improve make parallel jobs support

* document the --parallel option

* disable the output-sync for interactive targets

platforms/arm_atsam/flash.mk

@@ -5,7 +5,7 @@
 
 flash: bin
 ifneq ($(strip $(PROGRAM_CMD)),)
-	$(PROGRAM_CMD)
+	$(UNSYNC_OUTPUT_CMD) && $(PROGRAM_CMD)
 else
 	$(PRINT_OK); $(SILENT) || printf "$(MSG_FLASH_ARCH)"
 endif