Code size report:

   bare-arm:    +0 +0.000% 
minimal x86:    +0 +0.000% 
   unix x64:    +0 +0.000% standard
      stm32:    +0 +0.000% PYBV10
     mimxrt:    +0 +0.000% TEENSY40
        rp2:    +0 +0.000% RPI_PICO_W
       samd:    +0 +0.000% ADAFRUIT_ITSYBITSY_M4_EXPRESS
  qemu rv32:    +0 +0.000% VIRT_RV32

In principle this looks reasonable, although I have two concerns:

1. It requires board header files to correctly use pico_cmake_set_default PICO_FLASH_SIZE_BYTES, and not all of them do (the pico-sdk has some magic to grep for these strings and then expose the settings to cmake...). Eg NULLBITS_BIT_C_PRO doesn't have that definition and doesn't build with this PR.
2. Eventually I want to auto-detect the size of the SPI flash, and allow the user to specify the size of filesystems (ROMFS, littlefs, FAT, etc) using a partition table. That might require a bit of trickery in the linker script, similar to what's done here.

(1) is pretty easy to fix for the boards in this repo (there's also POLOLU_ZUMO_2040_ROBOT, WEACTSTUDIO and WEACTSTUDIO_RP2350B_CORE to fix), although anyone with a custom board outside this repo might also need to add such a definition.

(2) is probably not an issue but worth considering.

@dpgeorge How about this? If PICO_FLASH_SIZE_BYTES is not set (actually, it appears to get set to ""), then it uses the same defaults as before (2MB for RP2040 or 4MB for RP2350).

IMO the default probably doesn't need to be different between the RP2040 and RP2350, but I'm just keeping the same behavior as before.

Squashed, rebased and merged in 6bfb83e

Hmm, this check is not working for me:

This does seem to work though:

if(DEFINED PICO_FLASH_SIZE_BYTES)

This statement:

If PICO_FLASH_SIZE_BYTES is not set (actually, it appears to get set to "")

appears to be wrong. Not sure if something changed since I last looked at this, but I tested a few boards, and if(DEFINED PICO_FLASH_SIZE_BYTES) seems to work fine.

I didn't catch this until now, because my project's firmware size dropped below 4MB shortly after this PR, and only just came back up over 4MB.