shared/readline: Use MP_REGISTER_ROOT_POINTER().

This uses MP_REGISTER_ROOT_POINTER() to register the readline_history root
pointer array used by shared/readline.c and removes the registration from
all mpconfigport.h files.

This also required adding a new MICROPY_READLINE_HISTORY_SIZE config option
since not all ports used the same sized array.

Signed-off-by: David Lechner <david@pybricks.com>

ports/minimal/Makefile

@@ -68,6 +68,7 @@ SRC_C += $(SRC_MOD)
 
 SRC_CXX += $(SRC_MOD_CXX)
 
+SRC_QSTR += shared/readline/readline.c
 SRC_QSTR += $(SRC_MOD) $(SRC_MOD_CXX)
 
 OBJ += $(PY_CORE_O) $(addprefix $(BUILD)/, $(SRC_C:.c=.o))

ports/minimal/mpconfigport.h

@@ -41,6 +41,3 @@ typedef long mp_off_t;
 #endif
 
 #define MP_STATE_PORT MP_STATE_VM
-
-#define MICROPY_PORT_ROOT_POINTERS \
-    const char *readline_hist[8];