Add support for compiling plain AVR C files

To compile plain AVR C files, the variable NO_CORE should be initialized
to a non-empty value.

When NO_CORE is set, the following changes happen
- boards.txt file is not parsed. The user should explicitly set MCU,
  F_CPU, fuse bits etc.
- MONITOR_BAUDRATE is not calculated

Fix #63

HISTORY.md

@@ -9,6 +9,7 @@ The following is the rough list of changes that went into different versions. I
 - Add a warning when HEX_MAXIMUM_SIZE is not specified
 - Add the ability to configure avrdude options. Fix issue #53
 - Handle cases where certain fuse bits are not present. Fix issue #61
+- Add support for compiling plain AVR C files. Fix issue #63
 
 ### 0.10.6 (2013-06-14)
 - Fix whitespace and add /dev/null redirection (https://github.com/sej7278)

arduino-mk/Arduino.mk

@@ -431,6 +431,10 @@ ifndef PARSE_BOARD_OPTS
     PARSE_BOARD_OPTS = --boards_txt=$(BOARDS_TXT)
 endif
 
+# If NO_CORE is set, then we don't have to parse boards.txt file
+# But the user might have to define MCU, F_CPU etc
+ifeq ($(strip $(NO_CORE)),)
+
     ifndef PARSE_BOARD_CMD
         PARSE_BOARD_CMD = $(PARSE_BOARD) $(PARSE_BOARD_OPTS)
     endif
@@ -474,10 +478,6 @@ ifndef ISP_LOCK_FUSE_PRE
         ISP_LOCK_FUSE_PRE  = $(shell $(PARSE_BOARD_CMD) $(BOARD_TAG) bootloader.unlock_bits)
     endif
 
-ifndef ISP_LOCK_FUSE_POST
-    ISP_LOCK_FUSE_POST = $(shell $(PARSE_BOARD_CMD) $(BOARD_TAG) bootloader.lock_bits)
-endif
-
     ifndef ISP_HIGH_FUSE
         ISP_HIGH_FUSE      = $(shell $(PARSE_BOARD_CMD) $(BOARD_TAG) bootloader.high_fuses)
     endif
@@ -490,15 +490,21 @@ ifndef ISP_EXT_FUSE
         ISP_EXT_FUSE       = $(shell $(PARSE_BOARD_CMD) $(BOARD_TAG) bootloader.extended_fuses)
     endif
 
-# Everything gets built in here (include BOARD_TAG now)
-ifndef OBJDIR
-	OBJDIR  	  = build-$(BOARD_TAG)
+    ifndef ISP_LOCK_FUSE_POST
+        ISP_LOCK_FUSE_POST = $(shell $(PARSE_BOARD_CMD) $(BOARD_TAG) bootloader.lock_bits)
     endif
 
     ifndef HEX_MAXIMUM_SIZE
         HEX_MAXIMUM_SIZE  = $(shell $(PARSE_BOARD_CMD) $(BOARD_TAG) upload.maximum_size)
     endif
 
+endif
+
+# Everything gets built in here (include BOARD_TAG now)
+ifndef OBJDIR
+    OBJDIR = build-$(BOARD_TAG)
+endif
+
 ########################################################################
 # Local sources
 #
@@ -516,11 +522,23 @@ LOCAL_OBJ_FILES = $(LOCAL_C_SRCS:.c=.o)   $(LOCAL_CPP_SRCS:.cpp=.o) \
 		$(LOCAL_INO_SRCS:.ino=.o) $(LOCAL_AS_SRCS:.S=.o)
 LOCAL_OBJS      = $(patsubst %,$(OBJDIR)/%,$(LOCAL_OBJ_FILES))
 
+# If NO_CORE is not set, then we need exactly one .pde or .ino file
+ifeq ($(strip $(NO_CORE)),)
+
+    ifeq ($(words $(LOCAL_PDE_SRCS) $(LOCAL_INO_SRCS)), 0)
+        ifeq ($(strip $(NO_CORE)),)
+            $(error No .pde or .ino files found. If you want to compile .c or .cpp files, then set NO_CORE)
+        endif
+    endif
+
+    # Ideally, this should just check if there are more than one file
     ifneq ($(words $(LOCAL_PDE_SRCS) $(LOCAL_INO_SRCS)), 1)
         #TODO: Support more than one file. https://github.com/sudar/Arduino-Makefile/issues/49
         $(error Need exactly one .pde or .ino file)
     endif
 
+endif
+
 # core sources
 ifeq ($(strip $(NO_CORE)),)
     ifdef ARDUINO_CORE_PATH
@@ -558,6 +576,7 @@ endif
 # to the command we're using (here screen). So, read the screen docs
 # for more information (search for 'character special device').
 #
+ifeq ($(strip $(NO_CORE)),)
     ifndef MONITOR_BAUDRATE
         SPEED = $(shell egrep -h 'Serial.begin\([0-9]+\)' $(LOCAL_PDE_SRCS) $(LOCAL_INO_SRCS) | sed -e 's/[^0-9]//g'| head -n1)
         MONITOR_BAUDRATE = $(findstring $(SPEED),300 1200 2400 4800 9600 14400 19200 28800 38400 57600 115200)
@@ -575,6 +594,7 @@ endif
     ifndef MONITOR_CMD
         MONITOR_CMD = screen
     endif
+endif
 
 ########################################################################
 # Include file to use for old .pde files