This switches stage2 to C and uses Jinja to change the C code based
on flash settings from https://github.com/adafruit/nvm.toml. It
produces the fastest settings for the given set of external flashes.
Flash size is no longer hard coded so switching flashes with similar
capabilities but different sizes should *just work*.
This PR also places "ITCM" code in RAM to save the XIP cache for
code execution. Further optimization is possible. A blink code.py
still requires a number of flash fetches every blink.
Fixes#4041
Presumably, switching it to generic hurt performance a bit.
I verified that the build-raspberry_pi_pico/boot2_padded_checksummed.S
built file has the same checksum as the old
bs2_default_padded_checksummed.S
Add SMPS mode pin to Raspi Pico pins.c; see section "4.3. Using the ADC" of the Pico datasheet for discussion.
Driving this pin high forces the onboard regulator into a lower noise PWM mode.
The RP2040 is new microcontroller from Raspberry Pi that features
two Cortex M0s and eight PIO state machines that are good for
crunching lots of data. It has 264k RAM and a built in UF2
bootloader too.
Datasheet: https://pico.raspberrypi.org/files/rp2040_datasheet.pdf
