Proposal: Improve Semihosting EXIT Behavior

[Marek V. <mar...@co...>]

Hello all,

I'd like to propose improvements to OpenOCD's semihosting behavior for 
the SYS_EXIT and SYS_EXIT_EXTENDED syscalls.

Currently, if GDB is not connected and the target calls `SYS_EXIT` or 
`SYS_EXIT_EXTENDED` (with the appropriate parameter):

  * If GDB is not connected - OpenOCD calls `exit(code)` immediately,
    without any cleanup. I believe this should be fixed - OpenOCD should
    go trough the standard shutdown sequence.
  * If GDB is connected - message is printed and target is either halted
    or resumed depending on `arm semihosting_resexit`.

I would like to make these improvements:

1. Avoid the direct call to `exit()`

2. Introduce a new command: `arm semihosting_exit_behavior`

The options would be:

  * `always_halt`
  * `always_exit`
  * `exit_if_no_gdb`

3. Introduce commands to get the program exit code in Tcl scripts:

  * `arm semihosting_has_program_exited` - would return true if the
    semihosting exit already happened
  * `arm semihosting_exit_code` - returns exit code (from
    `SYS_EXIT_EXTENDED`)

Would you be in favor of this proposal? I welcome any comments.

Thank you,

Marek Vrbka