Allow transports to override the selected target (hla configs unification)

Allow transports to override the selected target (hla configs unification)

This should allow to share common configs for both regular access and
high-level adapters.

Use the newly-added functionality in stlink and icdi drivers, amend
the configs accordingly.

Runtime-tested with a TI tm4c123g board.

Change-Id: Ibb88266a4ca25f06f6c073e916c963f017447bad
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
[gus@projectgus.com: context-specific deprecation warnings]
Signed-off-by: Angus Gratton <gus@projectgus.com>
[andrew.smirnov@gmail.com: additional nrf51.cfg mods]
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Tested-by: Andrey Skvortsov <andrej.skvortzov@gmail.com>
Reviewed-on: http://openocd.zylin.com/1664
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>