Erase value on nor flash

[rmja]

It would be really useful if the erase value of a flash was exposed on the NorFlash trait(s). Maybe with a default of 0xFF, something like:

trait NorFlash {
    ...
    const ERASE_VALUE: u8 = 0xFF;
}

[chrysn]

This is part oft the larger discussion at #23. That one has stalled, maybe your fresh pair of eyes could help get that unstuck.

[kingofpayne]

I also believe that would be a good feature.
This would allow embassy-boot to be compatible with more devices (it currently fails on STM32L0 for instance).
As the comment says, they expect such a feature in https://github.com/embassy-rs/embassy/blob/3d6a270f30c45eaf394c8eb8bf182dd1a7ec2d7b/embassy-boot/src/lib.rs#L17