Add the "-semihosting-config" option.

Add the "-semihosting-config" option.

The usual semihosting behaviour is to process the system calls locally and
return; unfortuantelly the initial implementation dinamically changed the
target to GDB during debug sessions, which, for the usual arm-none-eabi-gdb,
is not implemented. The result was that during debug sessions the semihosting
calls were discarded.

This patch adds a configuration variable and an option to set it on the
command line:

    -semihosting-config [enable=on|off,]target=native|gdb|auto

This option enables semihosting and defines where the semihosting calls will
be addressed, to QEMU ('native') or to GDB ('gdb'). The default is auto, which
means 'gdb' during debug sessions and 'native' otherwise.

Signed-off-by: Liviu Ionescu <ilg@livius.net>
Message-id: 1416341957-9796-1-git-send-email-ilg@livius.net
[PMM: moved declaration and definition of semihosting_target to
 gdbstub.h and gdbstub.c to fix build failure on linux-user]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>