Support for semihosting

[notjob]

It would be nice to allow support for semihosting for targets that have no serial output possibilities. In order to achieve that, one needs to add --specs=rdimon.specs to the linker options. But then since the low-level functions defined in rdimon.specs are already defined in retarget.cpp, the program will not compile or work. These functions are:

• _lseek
• _isatty
• _open
• _close

I can see that in device.h file of some mbed-hal-* modules, DEVICE_SEMIHOST exists but it doesn't seem to have been used at all. One suggestion that I've tried and worked is to #if !DEVICE_SEMIHOST the definitions of those functions out. The two functions below are also necessary to exclude for semihosting to work correctly:

• _read
• _write

[ciarmcom]

ARM Internal Ref: IOTSFW-2603

[Kukunin]

Spent at least 4 hours trying to enable semihosting in my mbed-based project.
Eventually figured out, that mbed implements low-level functions strictly to redirect them to UART.
So, 👍 for this issue

[amq]

Just noticed https://github.com/ARMmbed/swo

It's not exactly printf() or tr_debug(), but it at least allows SWO output for devices without STDIO_MESSAGES