reform2-lpc-fw: write out descriptive error if REFORM_MOTHERBOARD_REV is missing

reform2-lpc-fw/src/boards/reform2/board_reform2.c

@@ -43,6 +43,10 @@
 // if you have a motherboard made before 2023 (2.0 R-3):
 //#define REFORM_MOTHERBOARD_REV REFORM_MBREV_20_R3
 
+#ifndef REFORM_MOTHERBOARD_REV
+#  error You must have REFORM_MOTHERBOARD_REV set to the firmware you intend to build
+#endif
+
 //#define REF2_DEBUG 1
 #define FW_STRING1 "MREF2LPC"
 #if REFORM_MOTHERBOARD_REV == 2