Makefile, set MCU at top and remove -nostdlib

Define MCU in one place at the top of the file.  Also allow it to be
overridden by the command line to compile for a different MCU without
changing the Makefile.
Reduce confusion, remove -nostdlib as it isn't obvious that it is
only being passed when compiling (which isn't linking so doesn't use
it), and isn't being passed to the linker (where it would fail if it
was, because routines from the library are used).

teensy3/Makefile

@@ -1,3 +1,9 @@
+# set your MCU type here, or make command line `make MCU=MK20DX256`
+MCU=MK20DX256
+
+# make it lower case
+LOWER_MCU := $(subst A,a,$(subst B,b,$(subst C,c,$(subst D,d,$(subst E,e,$(subst F,f,$(subst G,g,$(subst H,h,$(subst I,i,$(subst J,j,$(subst K,k,$(subst L,l,$(subst M,m,$(subst N,n,$(subst O,o,$(subst P,p,$(subst Q,q,$(subst R,r,$(subst S,s,$(subst T,t,$(subst U,u,$(subst V,v,$(subst W,w,$(subst X,x,$(subst Y,y,$(subst Z,z,$(MCU)))))))))))))))))))))))))))
+MCU_LD = $(LOWER_MCU).ld
 
 # The name of your project (used to name the compiled .hex file)
 TARGET = main
@@ -10,7 +16,7 @@ ARDUINOPATH ?= ../../../../..
 OPTIONS = -DF_CPU=48000000 -DUSB_SERIAL -DLAYOUT_US_ENGLISH -DUSING_MAKEFILE
 
 # options needed by many Arduino libraries to configure for Teensy 3.0
-OPTIONS += -D__MK20DX256__ -DARDUINO=10600 -DTEENSYDUINO=121
+OPTIONS += -D__$(MCU)__ -DARDUINO=10600 -DTEENSYDUINO=121
 
 
 # Other Makefiles and project templates for Teensy 3.x:
@@ -40,7 +46,7 @@ COMPILERPATH = $(ARDUINOPATH)/hardware/tools/arm/bin
 #************************************************************************
 
 # CPPFLAGS = compiler options for C and C++
-CPPFLAGS = -Wall -g -Os -mcpu=cortex-m4 -mthumb -nostdlib -MMD $(OPTIONS) -I.
+CPPFLAGS = -Wall -g -Os -mcpu=cortex-m4 -mthumb -MMD $(OPTIONS) -I.
 
 # compiler options for C++ only
 CXXFLAGS = -std=gnu++0x -felide-constructors -fno-exceptions -fno-rtti
@@ -49,7 +55,7 @@ CXXFLAGS = -std=gnu++0x -felide-constructors -fno-exceptions -fno-rtti
 CFLAGS =
 
 # linker options
-LDFLAGS = -Os -Wl,--gc-sections,--defsym=__rtc_localtime=0 --specs=nano.specs -mcpu=cortex-m4 -mthumb -Tmk20dx256.ld
+LDFLAGS = -Os -Wl,--gc-sections,--defsym=__rtc_localtime=0 --specs=nano.specs -mcpu=cortex-m4 -mthumb -T$(MCU_LD)
 
 # additional libraries to link
 LIBS = -lm
@@ -72,7 +78,7 @@ OBJS := $(C_FILES:.c=.o) $(CPP_FILES:.cpp=.o)
 
 all: $(TARGET).hex
 
-$(TARGET).elf: $(OBJS) mk20dx256.ld
+$(TARGET).elf: $(OBJS) $(MCU_LD)
 	$(CC) $(LDFLAGS) -o $@ $(OBJS) $(LIBS)
 
 %.hex: %.elf
@@ -87,5 +93,3 @@ $(TARGET).elf: $(OBJS) mk20dx256.ld
 
 clean:
 	rm -f *.o *.d $(TARGET).elf $(TARGET).hex
-
-