ccid: Introduce ccid_slot_ops + implement simulator/stub for it

This adds a new interface to the CCID implementation, on the card/slot
side.  The purpose of this interface (based on function pointers) is
to allow for different real hardware or virtual implementations.

Change-Id: I2c38aa69594a3b22bb5b5e256edfb48481e42793
diff --git a/ccid/Makefile b/ccid/Makefile
index cd8321f..2194180 100644
--- a/ccid/Makefile
+++ b/ccid/Makefile
@@ -1,6 +1,6 @@
 CFLAGS=-Wall -g
 
-ccid_functionfs: ccid_main_functionfs.o ccid_proto.o ccid_device.o
+ccid_functionfs: ccid_main_functionfs.o ccid_proto.o ccid_device.o ccid_slot_sim.o
 	$(CC) $(CFLAGS) -lasan -losmocore -ltalloc -laio -o $@ $^
 
 %.o: %.c