Support for teleprobe "exports"

[plaes]

Currently teleprobe is mostly used as a remote CI tool, where one continuously flashes same devices with different firmwares - ie static devices and dynamic firmware.
Another scenario for teleprobe would be series testing/flashing of devices together with certain data-logging and calibration tests where one wants to save some device-specific information into database: ie chip-specific information like serial numbers and calibration results and also include data from companion chips (flashes, RTCs etc.).

I have a minimal draft PoC implemented in #20:

Demo:

 INFO  teleprobe::run > run_from_ram: true
 INFO  teleprobe::run > flashing program...
 INFO  teleprobe::run > flashing done!
 INFO  device         > 0.000000 Hello World!
 INFO  device         > 0.100128 slept for 100 ms
 INFO  device         > 0.100433 old: _TELEPROBE_INFO_VAR1: ptr = 0x20001732, val = [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] 
 INFO  device         > 0.101745 new: _TELEPROBE_INFO_VAR1: ptr = 0x20001732, val = [119, 48, 48, 116, 119, 48, 48, 116, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] 
 INFO  device         > 0.103210 Test OK
 INFO  teleprobe::run > Found: _TELEPROBE_INFO_VAR1 -> w00tw00t

From firmware side the changes are currently minimal:

1. Define a static mut array and make sure it's not demangled and kept in firmware:

#[used]
#[no_mangle]
static mut _TELEPROBE_INFO_VAR1: [u8; 64] = [0; 64];

2. Populate this array with data:

async fn main(..) {
    unsafe {
        let data = b"w00tw00t";
        _TELEPROBE_INFO_VAR1[..data.len()].copy_from_slice(data);
        // info!("_TELEPROBE_INFO_VAR1: ptr = {:?}, val = {:?} ", &mut _TELEPROBE_INFO_VAR1 as * mut _, _TELEPROBE_INFO_VAR1);
    }
}

Some ideas that I have:

1. Symbol autodiscovery instead of harcoded list - use .teleprobe.exports section and some macro magic from Rust side to populate the section with a list of exported variables (and ideally their addresses). (I tried to implement something, but my rust- and ld-foo isn't strong enough).
2. Client side vs server side handling - there's already some section parsing happening in the client side, so we should create a list of "required" symbols and pass this info to server.
3. Support for other data types than byte arrays (via symbol demangling?).