BSPs should handle rp2040_boot2 about rp-hal

[9names]

An alternative approach was suggested by adamgreig in matrix chat:
A default feature, that when enabled makes the BSPs link rp2040_boot2::BOOT_LOADER for you.
Pros:

• No extra boilerplate in every project, completely invisible to the user.

Cons:

• Being invisible means that if it stops working for whatever reason, it's more painful to fix
• Default features do not spark joy

[jannic]

One possible disadvantage (of both approaches):
The build tools necessary to build the bootloader (arm-none-eabi-gcc) may not be easily available everywhere. (Windows?)
In that case, one can easily include a precompiled boot2, instead:

#[link_section = ".boot2"]
#[used]
pub static BOOT2: [u8; 256] = *include_bytes!("boot2.bin");

But if rp2040-boot2-rs is a dependency of the BSP, one also needs to fork the BSP and remove that dependency.

(Please note that there is already a suggestion to change the boot2 crate so the assembler does not run automatically at build time: rp-rs/rp2040-boot2#2. With that change, my point would be void.)

[thejpster]

Nearly there!