Allow specification of EXST flash chip.

e.g.

```
make CONFIG=SPRACINGH7EXTREME EXST_FLASH_CHIP=W25N01GV
make CONFIG=SPRACINGH7EXTREME EXST_FLASH_CHIP=W25Q128JV
```

If flash chip is not specified, the target configured default one is
used.

Flash chip and load address is added to the build file.

e.g.

```
./obj/betaflight_4.2.0_SPRACINGH7EXTREME-W25N01GV_<revision>.bin
./obj/betaflight_4.2.0_SPRACINGH7EXTREME-W25Q128JV_<norevision>.bin
```

Makefile

@@ -25,6 +25,7 @@ OPTIONS   ?=
 
 # compile for External Storage Bootloader support
 EXST      ?= no
+EXST_FLASH_CHIP ?=
 
 # compile for target loaded into RAM
 RAM_BASED ?= no
@@ -48,6 +49,7 @@ SERIAL_DEVICE   ?= $(firstword $(wildcard /dev/ttyACM*) $(firstword $(wildcard /
 # Flash size (KB).  Some low-end chips actually have more flash than advertised, use this to override.
 FLASH_SIZE ?=
 
+
 ###############################################################################
 # Things that need to be maintained as the source changes
 #
@@ -316,6 +318,10 @@ ifeq ($(REV),yes)
 TARGET_NAME := $(TARGET_NAME)_$(REVISION)
 endif
 
+ifeq ($(EXST),yes)
+TARGET_NAME := $(TARGET_NAME)-$(EXST_FLASH_CHIP)
+endif
+
 TARGET_FULLNAME = $(FORKNAME)_$(FC_VER)_$(TARGET_NAME)
 #
 # Things we will build

src/main/target/STM32H750/target.mk

@@ -7,3 +7,24 @@ ifneq ($(EXST),yes)
 TARGET_FLASH_SIZE   := 1024
 LD_SCRIPT            = $(LINKER_DIR)/stm32_flash_h750_1m.ld
 endif
+
+ifeq ($(EXST),yes)
+# EXST flash chip configuration
+
+ifeq ($(EXST_FLASH_CHIP),)
+EXST_FLASH_CHIP = W25N01GV
+endif
+
+ifeq ($(EXST_FLASH_CHIP),W25N01GV)
+EXST_ADJUST_VMA = 0x97CE0000 # 128MB NAND flash
+else
+ifeq ($(EXST_FLASH_CHIP),W25Q128JV)
+EXST_ADJUST_VMA = 0x90F90000 # 16MB NOR flash
+endif
+endif
+
+#ifeq ($(EXST_ADJUST_VMA),)
+#EXST_ADJUST_VMA = 0x97CE0000
+#endif
+
+endif