semihosting: create SemihostingConfig structure and semihost.h

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 <[email protected]>
Reviewed-by: Peter Maydell <[email protected]>
Message-id: 1434643256[email protected]
Signed-off-by: Peter Maydell <[email protected]>