HAL_ChibiOS: support a app->bootloader comms region in SRAM

this allows for UAVCAN update without a 2nd BeginFirmwareUpdate call

libraries/AP_HAL_ChibiOS/hwdef/CUAV_GPS/hwdef-bl.dat

@@ -106,3 +106,6 @@ define BOOTLOADER_BAUDRATE 57600
 
 # use a small bootloader timeout
 define HAL_BOOTLOADER_TIMEOUT 1000
+
+# reserve 256 bytes for comms between app and bootloader
+RAM_RESERVE_START 256

libraries/AP_HAL_ChibiOS/hwdef/CUAV_GPS/hwdef.dat

@@ -168,3 +168,6 @@ define AP_PERIPH_LED_BRIGHT_DEFAULT 50
 
 # use the app descriptor needed by MissionPlanner for CAN upload
 env APP_DESCRIPTOR MissionPlanner
+
+# reserve 256 bytes for comms between app and bootloader
+RAM_RESERVE_START 256

libraries/AP_HAL_ChibiOS/hwdef/ZubaxGNSS/hwdef-bl.dat

@@ -29,6 +29,9 @@ FLASH_SIZE_KB 256
 # reserve space for params
 FLASH_RESERVE_END_KB 2
 
+# reserve 256 bytes for comms between app and bootloader
+RAM_RESERVE_START 256
+
 # USART3 for debug
 #STDOUT_SERIAL SD3
 #STDOUT_BAUDRATE 115200

libraries/AP_HAL_ChibiOS/hwdef/ZubaxGNSS/hwdef.dat

@@ -12,6 +12,9 @@ define CAN_APP_NODE_NAME "org.ardupilot.ap_periph_ZubaxGNSS"
 # bootloader starts firmware at 34k
 FLASH_RESERVE_START_KB 34
 
+# reserve 256 bytes for comms between app and bootloader
+RAM_RESERVE_START 256
+
 # store parameters in last 2 pages
 define STORAGE_FLASH_PAGE 126
 define HAL_STORAGE_SIZE 800

libraries/AP_HAL_ChibiOS/hwdef/f103-periph/hwdef-bl.dat

@@ -95,3 +95,6 @@ define HAL_BOOTLOADER_TIMEOUT 1000
 # and look for it low then we know user has pulled it down and
 # want to stay in the bootloader
 PB6 STAY_IN_BOOTLOADER INPUT FLOATING
+
+# reserve 256 bytes for comms between app and bootloader
+RAM_RESERVE_START 256

libraries/AP_HAL_ChibiOS/hwdef/f103-periph/hwdef.dat

@@ -142,3 +142,6 @@ define HAL_COMPASS_MAX_SENSORS 1
 
 # use the app descriptor needed by MissionPlanner for CAN upload
 env APP_DESCRIPTOR MissionPlanner
+
+# reserve 256 bytes for comms between app and bootloader
+RAM_RESERVE_START 256

libraries/AP_HAL_ChibiOS/hwdef/scripts/chibios_hwdef.py

@@ -613,6 +613,11 @@ def write_mcu_config(f):
     f.write('#define HAL_MEMORY_REGIONS %s\n' % ', '.join(regions))
     f.write('#define HAL_MEMORY_TOTAL_KB %u\n' % total_memory)
 
+    f.write('#define HAL_RAM0_START 0x%08x\n' % ram_map[0][0])
+    ram_reserve_start = get_config('RAM_RESERVE_START', default=0, type=int)
+    if ram_reserve_start > 0:
+        f.write('#define HAL_RAM_RESERVE_START 0x%08x\n' % ram_reserve_start)
+
     f.write('\n// CPU serial number (12 bytes)\n')
     f.write('#define UDID_START 0x%08x\n\n' % get_mcu_config('UDID_START', True))
 
@@ -696,15 +701,23 @@ def write_ldscript(fname):
 
     print("Generating ldscript.ld")
     f = open(fname, 'w')
+    ram0_start = ram_map[0][0]
+    ram0_len = ram_map[0][1] * 1024
+
+    # possibly reserve some memory for app/bootloader comms
+    ram_reserve_start = get_config('RAM_RESERVE_START', default=0, type=int)
+    ram0_start += ram_reserve_start
+    ram0_len -= ram_reserve_start
+
     f.write('''/* generated ldscript.ld */
 MEMORY
 {
     flash : org = 0x%08x, len = %uK
-    ram0  : org = 0x%08x, len = %uk
+    ram0  : org = 0x%08x, len = %u
 }
 
 INCLUDE common.ld
-''' % (flash_base, flash_length, ram_map[0][0], ram_map[0][1]))
+''' % (flash_base, flash_length, ram0_start, ram0_len))
 
 def copy_common_linkerscript(outdir, hwdef):
     dirpath = os.path.dirname(hwdef)