Merge pull request micropython#976 from mrmcwethy/makefile1

Test the BOARD parameter for nullness

Author: tannewt

ports/atmel-samd/Makefile

@@ -4,9 +4,12 @@ PYTHON3 ?= python3
 
 # Select the board to build for: if not given on the command line,
 # then default to PYBV10.
-BOARD ?= metro_m0_express
-ifeq ($(wildcard boards/$(BOARD)/.),)
-$(error Invalid BOARD specified)
+ifeq ($(BOARD),)
+  $(error You must provide a BOARD parameter)
+else
+  ifeq ($(wildcard boards/$(BOARD)/.),)
+    $(error Invalid BOARD specified)
+  endif
 endif