[rp20xx][pico-sdk] add some missing / generated headers

- sys/cdefs is wanted for __CONCAT()
- version.h and config_autogen.h are generated by the pico sdk
- config_autogen expects to include board file
- instead define PICO_xyz in target rules.mk

target/pico/rules.mk

@@ -5,7 +5,11 @@ MODULE := $(LOCAL_DIR)
 PLATFORM := rp20xx
 
 GLOBAL_DEFINES += \
-	TARGET_HAS_DEBUG_LED=1
+	TARGET_HAS_DEBUG_LED=1 \
+	PICO_FLASH_SPI_CLKDIV=2 \
+	PICO_FLASH_SIZE_BYTES=0x200000 \
+	PICO_FLOAT_SUPPORT_ROM_V1=1 \
+	PICO_DOUBLE_SUPPORT_ROM_V1=1
 
 MODULE_SRCS += \
 	$(LOCAL_DIR)/boot.stage2.S \