stm32/mboot: Allow a board to customise the linker scripts.

A board can now define MBOOT_LD_FILES (at the Makefile-level) to specify
custom linker scripts.  And stm32_generic.ld has been split into 2 pieces
so one or the other can be reused (usually stm32_sections.ld wolud be
reused by a board, and stm32_memory.ld redefined).

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

ports/stm32/mboot/Makefile

@@ -85,7 +85,8 @@ CFLAGS += -DBUILDING_MBOOT=$(BUILDING_MBOOT)
 CFLAGS += -DMICROPY_HW_STM32WB_FLASH_SYNCRONISATION=0
 CFLAGS += -DBOOTLOADER_DFU_USB_VID=$(BOOTLOADER_DFU_USB_VID) -DBOOTLOADER_DFU_USB_PID=$(BOOTLOADER_DFU_USB_PID)
 
-LDFLAGS = -nostdlib -L . -T stm32_generic.ld -Map=$(@:.elf=.map) --cref
+MBOOT_LD_FILES ?= stm32_memory.ld stm32_sections.ld
+LDFLAGS = -nostdlib -L . $(addprefix -T,$(MBOOT_LD_FILES)) -Map=$(@:.elf=.map) --cref
 LIBS = $(shell $(CC) $(CFLAGS) -print-libgcc-file-name)
 
 # Remove uncalled code from the final image.

ports/stm32/mboot/stm32_memory.ld

@@ -0,0 +1,10 @@
+/*
+    Linker script fragment for mboot on an STM32xxx MCU.
+    This defines the memory sections for the bootloader to use.
+*/
+
+MEMORY
+{
+    FLASH_BL (rx)   : ORIGIN = 0x08000000, LENGTH = 32K
+    RAM (xrw)       : ORIGIN = 0x20000000, LENGTH = 120K
+}

ports/stm32/mboot/stm32_generic.ld → ports/stm32/mboot/stm32_sections.ld

@@ -1,14 +1,8 @@
 /*
-    GNU linker script for generic STM32xxx MCU
+    Linker script fragment for mboot on an STM32xxx MCU.
+    This needs the following MEMORY sections to be defined: FLASH_BL, RAM.
 */
 
-/* Specify the memory areas */
-MEMORY
-{
-    FLASH_BL (rx)   : ORIGIN = 0x08000000, LENGTH = 32K /* sector 0 (can be 32K) */
-    RAM (xrw)       : ORIGIN = 0x20000000, LENGTH = 120K
-}
-
 /* produce a link error if there is not this amount of RAM for these sections */
 _minimum_stack_size = 8K;