Set PICO_FLASH_SPI_CLKDIV=2 when building boot2

The assembly files contain a configurable clock divider, with a default
value of 4. The C SDK overwrites this value with 2, eg. for the rp pico:

https://github.com/raspberrypi/pico-sdk/blob/master/src/boards/include/boards/pico.h#L70

Because we didn't set that value in rp2040-boot2-rs, rust firmwares
effectively ran the flash chip at half speed.

Therefore, set the preprocessor define from build.rs, to match what the
C SDK does.

Author: jannic

bin/boot2_at25sf128a.padded.bin

@@ -34,6 +34,7 @@ fn make_elf<P: AsRef<Path>, Q: AsRef<Path>>(input_path: P, out_dir: Q) -> PathBu
         .arg("-fPIC")
         .arg("--specs=nosys.specs")
         .arg("-Isrc/include")
+        .arg("-DPICO_FLASH_SPI_CLKDIV=2")
         .arg(input_path)
         .arg("-o")
         .arg(&result_path)