Fix tools/build_all_headers.py to cope with chip-specific headers

[lurch]

Fixes #2220

With SORT_HEADERS_BY_DIRECTORY = True it shows you which directory the include files came from (which is the same behaviour as previous versions of this script) with the #if PICO_RP2040 and #if PICO_RP2350 headers at the very end

// rp2_common/hardware_pio/include
#include "hardware/pio.h"
#include "hardware/pio_instructions.h"

// rp2_common/hardware_pll/include
#include "hardware/pll.h"

// rp2_common/hardware_powman/include
#include "hardware/powman.h"

// rp2_common/hardware_pwm/include
#include "hardware/pwm.h"

// rp2_common/hardware_rcp/include
#include "hardware/rcp.h"

// rp2_common/hardware_resets/include
#include "hardware/resets.h"
...
// rp2xxx/hardware_structs/include
...
#include "hardware/structs/io_bank0.h"
#include "hardware/structs/io_qspi.h"
#include "hardware/structs/iobank0.h"
#include "hardware/structs/ioqspi.h"
#include "hardware/structs/mpu.h"
#include "hardware/structs/nvic.h"
#include "hardware/structs/pads_bank0.h"
#include "hardware/structs/pads_qspi.h"
#include "hardware/structs/padsbank0.h"
#include "hardware/structs/pio.h"
#include "hardware/structs/pll.h"

but with SORT_HEADERS_BY_DIRECTORY = False it just outputs a "flattened" list (which more closely matches the big list of header files in https://github.com/raspberrypi/pico-sdk/blob/develop/test/kitchen_sink/kitchen_sink.c ) with the #if PICO_RP2040 and #if PICO_RP2350 headers intermingled with everything else

#include "hardware/pio.h"
#include "hardware/pio_instructions.h"
#include "hardware/platform_defs.h"
#include "hardware/pll.h"
#include "hardware/powman.h"
#include "hardware/pwm.h"
#include "hardware/rcp.h"
#include "hardware/resets.h"
...
#include "hardware/structs/io_bank0.h"
#include "hardware/structs/io_qspi.h"
#include "hardware/structs/iobank0.h"
#include "hardware/structs/ioqspi.h"
#if PICO_RP2040
#include "hardware/structs/m0plus.h"
#endif
#if PICO_RP2350
#include "hardware/structs/m33.h"
#include "hardware/structs/m33_eppb.h"
#endif
#include "hardware/structs/mpu.h"
#include "hardware/structs/nvic.h"
#if PICO_RP2350
#include "hardware/structs/otp.h"
#endif
#include "hardware/structs/pads_bank0.h"
#include "hardware/structs/pads_qspi.h"
#include "hardware/structs/padsbank0.h"
#include "hardware/structs/pio.h"
#include "hardware/structs/pll.h"