semihosting: create SemihostingConfig structure and semihost.h

Remove semihosting_enabled and semihosting_target and replace them with
SemihostingConfig structure containing equivalent fields. The structure
is defined in vl.c where it is actually set.

Also introduce separate header file include/exec/semihost.h allowing to
access semihosting config related stuff from target specific semihosting
code.

Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1434643256-16858-2-git-send-email-leon.alrae@imgtec.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>