Add mpconfigport.mk file to configure which modules to include into build.

Proof of concept, controls "ffi" module as one which requires external
dependencies.

unix/Makefile

@@ -1,4 +1,5 @@
 include ../py/mkenv.mk
+-include mpconfigport.mk
 
 # define main target
 PROG = micropython
@@ -10,9 +11,16 @@ QSTR_DEFS = qstrdefsport.h
 include ../py/py.mk
 
 # compiler settings
-CFLAGS = -I. -I$(PY_SRC) -Wall -Werror -ansi -std=gnu99 -DUNIX
-CFLAGS += -I/usr/lib/libffi-3.0.13/include
-LDFLAGS = -lm -ldl -lffi
+CFLAGS = -I. -I$(PY_SRC) -Wall -Werror -ansi -std=gnu99 -DUNIX $(CFLAGS_MOD)
+LDFLAGS = $(LDFLAGS_MOD) -lm
+
+ifeq ($(MICROPY_MOD_FFI),1)
+# Note - include path below is specific to @dpgeorge
+CFLAGS_MOD += -I/usr/lib/libffi-3.0.13/include -DMICROPY_MOD_FFI=1
+LDFLAGS_MOD += -ldl -lffi
+SRC_MOD += ffi.c
+endif
+
 
 # Debugging/Optimization
 ifdef DEBUG
@@ -26,7 +34,7 @@ SRC_C = \
 	main.c \
 	file.c \
 	socket.c \
-	ffi.c \
+	$(SRC_MOD)
 
 OBJ = $(PY_O) $(addprefix $(BUILD)/, $(SRC_C:.c=.o))
 LIB = -lreadline

unix/main.c

@@ -242,7 +242,9 @@ int main(int argc, char **argv) {
 
     file_init();
     rawsocket_init();
+#if MICROPY_MOD_FFI
     ffi_init();
+#endif
 
     // Here is some example code to create a class and instance of that class.
     // First is the Python, then the C code.

unix/mpconfigport.mk

@@ -0,0 +1,4 @@
+# Enable/disable modules to be included in interpreter
+
+# ffi module requires libffi (libffi-dev Debian package)
+MICROPY_MOD_FFI = 0