Uses GPIO.E2 (Request To Send) to detect the holdswitch status,
it is a temporal workaround that seems to work on all models.
Holdswitch status must be detected to drive low GPIO.E2 (RTS)
and GPIO.E4 (Data Out) when the holdswitch is locked, otherwise
battery life decreases about 25%.
Holdswitch unlock action is detected by reading the HELLO message
that the external wheel controller sends when it is powered on,
this allows to quickly capture clickwheel activity after unlock.
GPIO.E2 is also used in case the HELLO message is missed because
the holdswitch was unlocked before Rockbox/bootloader starts.
These 2 lines (RTS and DOUT) can not be used to transmit messages
to the external clickwheel controller, not a problem, actually no
messages are sent while normal operation, only at initialization
stage.
Change-Id: I415fe54bfcbc2086d0f56d7affe6f789ce81a6db
348bfc5c8f