stm32f4: Add option erase command

--
Uwe Bonnes                bon@elektron.ikp.physik.tu-darmstadt.de

Institut fuer Kernphysik  Schlossgartenstrasse 9  64289 Darmstadt
--------- Tel. 06151 162516 -------- Fax. 06151 164321 ----------
>From d488dffe4dab4e5188c750489006260745a9e25a Mon Sep 17 00:00:00 2001
From: Uwe Bonnes <bon@elektron.ikp.physik.tu-darmstadt.de>
Date: Mon, 15 Apr 2013 15:26:58 +0200
Subject: stm32f4: Add option erase command
Merge branch 'master' into hostio
semihosting: improve handling of console I/O

This implements special handling for SYS_OPEN to catch requests for
":tt" which is reserved by ARM for console input/output. They're
mapped to the appropriate GDB file descriptors automatically.

An additional file handle offset is introduced because ARM doesn't
consider zero handle to be valid.

Signed-off-by: Paul Fertser <fercerpav@gmail.com>