Fix default BOARD setting & messages

It's important that these lines NOT be indented with tabs, because
that provokes Make to say that commands appear before a target.

Author: jepler

ports/broadcom/Makefile

@@ -1,12 +1,12 @@
 # Select the board to build for.
-BOARD=raspberrypi_pi4
+BOARD?=raspberrypi_pi4b
 
 ifeq ($(BOARD),)
-	$(error You must provide a BOARD parameter)
+  $(error You must provide a BOARD parameter)
 else
-	ifeq ($(wildcard boards/$(BOARD)/.),)
-		$(error Invalid BOARD specified)
-	endif
+  ifeq ($(wildcard boards/$(BOARD)/.),)
+    $(error Invalid BOARD "$(BOARD)" specified)
+  endif
 endif
 
 # If the build directory is not given, make it reflect the board name.