Jira 890, Allow BLE Descriptor reading to be blocking, git 478

[sgbihu]

Feature requested:
To make the reading of the Descriptor of a BLE Characteristic a blocking read method. The mechanism is similar to the the Characteristic read method. However, for Descriptor read(), it is always a blocking read.

Code Mods:

1. libraries/CurieBLE/src/internal/BLEDescriptorImp.cpp:
   • Added a flag for the Descriptor read completion event to set in foreground.
   • Check on the Descriptor read completion event in the background as an exit condition for the read method.

[SidLeung]

@bigdinotech @eriknyquist , please review the code change.

@russmcinnis @noelpaz , please perform system testing.

This feature is requested for Deneb.RC2 release.

[russmcinnis]

System test passed PR490(471, 477, 480, 484 and 489). will unit test next.

[russmcinnis]

This has been system tested. The peripheral_explorer sketch was able to read() a descriptor successfully without the delay(1000). The read() took 95 msec. I can get some traces tomorrow. @SidLeung can see if this looks good enough.

[SidLeung]

@russmcinnis , please save your traces in the Jira ticket for record.