cfg: Added st nucleo board configs.

This patch adds board configs for Nucleo FXXX boards.

Change-Id: I25bfd7d63f734bad710917efa67bc6096aa60ef8
Signed-off-by: Jens Hoffmann <jehoffma@gmail.com>
gdb_server: Fix segfault in (and rewrite) decode_xfer_read

Introduced by 537b06a81 (free non-malloced memory).

Rewrite to use standard C string routines and make returning annex
optional since it's not currently used.

Change-Id: Idf3698a482dfeff7fa5ea1660fd89122eb80b68d
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>