gpio.c: Added ISR thread termination flag

Also modified ISR handler logic to account for the flag.

This fixes isrExit() deadlock in Python interactive mode
and prevents a more generic problem of a spurious ISR call
after mraa_gpio_isr_exit() is run.

Closes #268.

Signed-off-by: Alex Tereschenko <alext.mkrs@gmail.com>
Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

include/mraa_internal_types.h

@@ -47,6 +47,7 @@ struct _gpio {
     void *isr_args; /**< args return when interupt service request triggered */
     pthread_t thread_id; /**< the isr handler thread id */
     int isr_value_fp; /**< the isr file pointer on the value */
+    mraa_boolean_t isr_thread_terminating; /**< is the isr thread being terminated? */
     mraa_boolean_t owner; /**< If this context originally exported the pin */
     mraa_result_t (*mmap_write) (mraa_gpio_context dev, int value);
     int (*mmap_read) (mraa_gpio_context dev);