HAL_ChibiOS: added option to limit size of bootloader

this will ensure our F4 bootloaders stay under 16k

libraries/AP_HAL_ChibiOS/hwdef/revo-mini/hwdef-bl.dat

@@ -13,6 +13,9 @@ STM32_PLLM_VALUE 8
 
 FLASH_SIZE_KB 1024
 
+# don't allow bootloader to use more than 16k
+FLASH_USE_MAX_KB 16
+
 USB_STRING_PRODUCT "ArduPilot-revo-BL"
 
 # bootloader is installed at zero offset

libraries/AP_HAL_ChibiOS/hwdef/scripts/chibios_hwdef.py

@@ -447,7 +447,9 @@ def write_mcu_config(f):
 
 def write_ldscript(fname):
     '''write ldscript.ld for this board'''
-    flash_size = get_config('FLASH_SIZE_KB', type=int)
+    flash_size = get_config('FLASH_USE_MAX_KB', type=int, default=0)
+    if flash_size == 0:
+        flash_size = get_config('FLASH_SIZE_KB', type=int)
 
     # space to reserve for bootloader and storage at start of flash
     flash_reserve_start = get_config(