stm32/flashbdev: Support generic flash storage config via link symbols.

A board can now define the following linker symbols to configure its flash
storage layout:

    _micropy_hw_internal_flash_storage_start
    _micropy_hw_internal_flash_storage_end
    _micropy_hw_internal_flash_storage_ram_cache_start
    _micropy_hw_internal_flash_storage_ram_cache_end

And optionally have a second flash segment by configuring
MICROPY_HW_ENABLE_INTERNAL_FLASH_STORAGE_SEGMENT2 to 1 and defining:

    _micropy_hw_internal_flash_storage2_start
    _micropy_hw_internal_flash_storage2_end

Signed-off-by: Damien George <damien@micropython.org>

ports/stm32/flashbdev.c

@@ -133,11 +133,35 @@ extern uint8_t _ram_fs_cache_end[];
 #define FLASH_MEM_SEG1_NUM_BLOCKS ((&_flash_fs_end - &_flash_fs_start) / 512)
 
 #else
-#error "no internal flash storage support for this MCU"
+
+// Generic configuration where the linker script specifies flash storage and RAM cache locations.
+
+extern uint8_t _micropy_hw_internal_flash_storage_start;
+extern uint8_t _micropy_hw_internal_flash_storage_end;
+extern uint8_t _micropy_hw_internal_flash_storage2_start;
+extern uint8_t _micropy_hw_internal_flash_storage2_end;
+extern uint8_t _micropy_hw_internal_flash_storage_ram_cache_start[];
+extern uint8_t _micropy_hw_internal_flash_storage_ram_cache_end[];
+
+#define CACHE_MEM_START_ADDR \
+    ((uintptr_t)&_micropy_hw_internal_flash_storage_ram_cache_start[0])
+#define FLASH_SECTOR_SIZE_MAX \
+    (&_micropy_hw_internal_flash_storage_ram_cache_end[0] - &_micropy_hw_internal_flash_storage_ram_cache_start[0])
+#define FLASH_MEM_SEG1_START_ADDR \
+    ((long)&_micropy_hw_internal_flash_storage_start)
+#define FLASH_MEM_SEG1_NUM_BLOCKS \
+    ((&_micropy_hw_internal_flash_storage_end - &_micropy_hw_internal_flash_storage_start) / 512)
+
+#if MICROPY_HW_ENABLE_INTERNAL_FLASH_STORAGE_SEGMENT2
+#define FLASH_MEM_SEG2_START_ADDR \
+    ((long)&_micropy_hw_internal_flash_storage2_start)
+#define FLASH_MEM_SEG2_NUM_BLOCKS \
+    ((&_micropy_hw_internal_flash_storage2_end - &_micropy_hw_internal_flash_storage2_start) / 512)
+#endif
+
 #endif
 
 #if !defined(FLASH_MEM_SEG2_START_ADDR)
-#define FLASH_MEM_SEG2_START_ADDR (0) // no second segment
 #define FLASH_MEM_SEG2_NUM_BLOCKS (0) // no second segment
 #endif
 
@@ -220,9 +244,11 @@ static uint32_t convert_block_to_flash_addr(uint32_t block) {
     if (block < FLASH_MEM_SEG1_NUM_BLOCKS) {
         return FLASH_MEM_SEG1_START_ADDR + block * FLASH_BLOCK_SIZE;
     }
+    #ifdef FLASH_MEM_SEG2_START_ADDR
     if (block < FLASH_MEM_SEG1_NUM_BLOCKS + FLASH_MEM_SEG2_NUM_BLOCKS) {
         return FLASH_MEM_SEG2_START_ADDR + (block - FLASH_MEM_SEG1_NUM_BLOCKS) * FLASH_BLOCK_SIZE;
     }
+    #endif
     // can add more flash segments here if needed, following above pattern
 
     // bad block

ports/stm32/mpconfigboard_common.h

@@ -57,6 +57,11 @@
 #define MICROPY_HW_ENABLE_INTERNAL_FLASH_STORAGE (1)
 #endif
 
+// If internal flash storage is enabled, whether to use a second segment of flash.
+#ifndef MICROPY_HW_ENABLE_INTERNAL_FLASH_STORAGE
+#define MICROPY_HW_ENABLE_INTERNAL_FLASH_STORAGE_SEGMENT2 (0)
+#endif
+
 // Whether to enable the RTC, exposed as pyb.RTC
 #ifndef MICROPY_HW_ENABLE_RTC
 #define MICROPY_HW_ENABLE_RTC (0)