* Platform updates
- Add PICO_RP2040=1 to rp2040 builds
- Add new PICO_RP2040_B0/1/2_SUPPORTED macros and retailer chip specific code to use
- Add doxygen to platform.h
- Make pico.h includable from assembly (because header order is important and tricky) - split out platform_asm.h
- Switch to using PICO_RP2040_B0_SUPPORTED in board headers
* pico_stdio_usb: be more explicit about includes, fix warning (#257)
* pico_base: NDEBUG backwards for absolute_time_t (#255)
* pico_util: missing extern C in queue.h (#249)
* build: remove -march which was masking -mcpu, now SVC available (#253)
* Add documentation for gpio_irq_callback_t and rtc_callback_t (fixes#175)
* Hook up pico_binary_info and pico_bootsel_via_double_reset to the Doxygen index
* Add link to new Raspberry Pi Pico FAQ
* Consistently add parentheses for \sa or \see links to function-names
* Use consistent capitalisation for 'GitHub'
* Small typos and markup errors
In C, func() is a function taking an unspecified number of arguments,
vs func(void) a function taking no arguments. In C++ both forms indicate
"no arguments."
Update these headers to use the (void) form, which is correct in both
languages and avoids complaints when -Wstrict-prototypes is specified.
