Implement "exported" variables allowing to pull data from firmware.

[plaes]

This implements basic groundwork for pulling data from firmware.

Demo:

 INFO  teleprobe::run > Found 2 exported variables: [
    0x2000c9a0,
    0x2000c9e0,
]
 INFO  teleprobe::run > Found match for addr 536922592 -> "_ZN5timer15_TELEPROBE_VAR217hd7ca27c6503da4d3E"
 INFO  teleprobe::run > Found match for addr 536922528 -> "_TELEPROBE_VAR1"
 INFO  teleprobe::run > run_from_ram: true
 INFO  teleprobe::run > flashing program...
 INFO  teleprobe::run > flashing done!
 INFO  device         > 0.101898 Test OK
 INFO  teleprobe::run > Found: _TELEPROBE_VAR1 -> yesyesno
 INFO  teleprobe::run > Found: _ZN5timer15_TELEPROBE_VAR217hd7ca27c6503da4d3E -> nonoyes!

Firmware-side changes (some "sugaring" would be eventually nice, but it works):

#[repr(C)]
union TPVar {
    f1: [u8; 64],
    f2: u32,
}

#[no_mangle]
static mut _TELEPROBE_VAR1: TPVar = TPVar { f1: [0; 64] };

// This works as well...
static mut _TELEPROBE_VAR2: TPVar = TPVar { f1: [0; 64] };

#[repr(C)]
pub struct TPPtr(*const u32);
unsafe impl Sync for TPPtr {}

#[link_section = ".teleprobe.export"]
#[used]
#[no_mangle]
static _TELEPROBE_EXPORTS: [TPPtr; 2] = [
    TPPtr(unsafe { &_TELEPROBE_VAR1.f2 as *const u32 }),
    TPPtr(unsafe { &_TELEPROBE_VAR2.f2 as *const u32 }),
];

fn main(...) {
    // Update our variables with custom data:
    unsafe {
        let data = b"yesyesno";
        _TELEPROBE_VAR1.f1[..data.len()].copy_from_slice(data);
        let data = b"nonoyes!";
        _TELEPROBE_VAR2.f1[..data.len()].copy_from_slice(data);

        info!(
            "new: _TELEPROBE_VAR1: ptr = {:?}, val = {:?} ",
            &mut _TELEPROBE_VAR1.f1 as *mut _, _TELEPROBE_VAR1.f1
        );
    }
}

[Dirbaio]

Thanks for the PR! Unfortunately, I feel this is outside teleprobe's scope. Scope is mainly running CI tests, where the firmware outputs just yes/no.

Production mass flashing is out of scope. If we decided it's in scope, there's a lot more features we'd have to have until this becomes a good mass flashing tool, and I just don't have the bandwidth for that.