Flash functions in PICO_NO_FLASH binaries

We assume that PICO_NO_FLASH binaries means the device does not have
a flash chip. But it's possible that you want a ram binary AND want
to mess about with flash, e.g. something that uses the unique flash id.

Add PICO_INCLUDE_FLASH_FUNCTIONS  which forces some flash functions to be
included.

Fixes #2796

src/rp2_common/hardware_flash/flash.c

@@ -248,7 +248,7 @@ void __no_inline_not_in_flash_func(flash_range_program)(uint32_t flash_offs, con
 //-----------------------------------------------------------------------------
 // Lower-level flash access functions
 
-#if !PICO_NO_FLASH
+#if !PICO_NO_FLASH || PICO_INCLUDE_FLASH_FUNCTIONS
 // Bitbanging the chip select using IO overrides, in case RAM-resident IRQs
 // are still running, and the FIFO bottoms out. (the bootrom does the same)
 static void __no_inline_not_in_flash_func(flash_cs_force)(bool high) {
@@ -334,7 +334,7 @@ void __no_inline_not_in_flash_func(flash_do_cmd)(const uint8_t *txbuf, uint8_t *
 static_assert(FLASH_UNIQUE_ID_SIZE_BYTES == FLASH_RUID_DATA_BYTES, "");
 
 void flash_get_unique_id(uint8_t *id_out) {
-#if PICO_NO_FLASH
+#if PICO_NO_FLASH && !PICO_INCLUDE_FLASH_FUNCTIONS
     __unused uint8_t *ignore = id_out;
     panic_unsupported();
 #else

src/rp2_common/hardware_flash/include/hardware/flash.h

@@ -52,6 +52,11 @@
 
 // PICO_CONFIG: PICO_FLASH_SIZE_BYTES, size of primary flash in bytes, type=int, default=Usually provided via board header, group=hardware_flash
 
+// PICO_CONFIG: PICO_INCLUDE_FLASH_FUNCTIONS, Force inclusion of flash functions in PICO_NO_FLASH binaries. Has no effect in flash binaries, type=bool, default=0, group=hardware_flash
+#ifndef PICO_INCLUDE_FLASH_FUNCTIONS
+#define PICO_INCLUDE_FLASH_FUNCTIONS 0
+#endif
+
 #ifdef __cplusplus
 extern "C" {
 #endif

src/rp2_common/pico_unique_id/unique_id.c

@@ -20,7 +20,7 @@ static pico_unique_board_id_t retrieved_id;
 
 static void __attribute__((PICO_UNIQUE_BOARD_ID_INIT_ATTRIBUTES)) _retrieve_unique_id_on_boot(void) {
 #if PICO_RP2040
-    #if PICO_NO_FLASH
+    #if PICO_NO_FLASH && !PICO_INCLUDE_FLASH_FUNCTIONS
         // The hardware_flash call will panic() if called directly on a NO_FLASH
         // build. Since this constructor is pre-main it would be annoying to
         // debug, so just produce something well-defined and obviously wrong.