add NextCheckKeyIsLong() and EnqueueKey() methods

NextCheckKeyIsLong() is called right before each call to CheckKey() to
tell the implementation if the key is a long-press or not.  (To be
used on devices with few buttons.)  It's done as a separate method
(rather than a parameter to CheckKey) to not break existing recovery
UI implementations.

EnqueueKey() can be called from CheckKey() to put arbitrary code codes
in the synchronous queue (to be processed by HandleMenuKey).

Change-Id: If8a83d66efe0bbc9e2dc178e5ebe12acd216324b