Platform updates (#611)

* 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

src/rp2_common/hardware_irq/irq_handler_chain.S

@@ -4,7 +4,7 @@
  * SPDX-License-Identifier: BSD-3-Clause
  */
 
-#include "hardware/platform_defs.h"
+#include "pico.h"
 #include "hardware/irq.h"
 
 #if !PICO_DISABLE_SHARED_IRQ_HANDLERS