F421: work around bug in old bootloader

the released F421 bootloader does this:

   0x08000b3a:	ldr	r0, [r4, am32-firmware#24]
   0x08000b3c:	str	r0, [r4, am32-firmware#28]
   0x08000b3e:	ldr	r0, [r5, #0]
   0x08000b40:	msr	MSP, r0
   0x08000b44:	ldr	r0, [r4, am32-firmware#28]
   0x08000b46:	ldmia.w	sp!, {r4, r5, r7, lr}
   0x08000b4a:	bx	r0

that writes to the application stack area before jumping. If the
application stack is set to the end of ram then the ldmia instruction
hard faults as it writes past the end of memory

this avoids the issue by only giving the app 15k instead of 16k of
ram. This is not needed when we fix the bootloader to use the right
jump method, which is this:

    // setup sp, msp and jump
    asm volatile(
        "mov sp, %0	\n"
        "msr msp, %0	\n"
        "bx	%1	\n"
	: : "r"(stack_top), "r"(JumpAddress) :);

Mcu/f421/AT32F421x6_FLASH.ld

@@ -20,8 +20,16 @@
 /* Entry Point */
 ENTRY(Reset_Handler)
 
-/* Highest address of the user mode stack */
-_estack = 0x20004000;    /* end of RAM */
+/*
+  Highest address of the user mode stack
+
+  we set this less than the 16k available due to a bug in the old
+  bootloader for F421. The old bootloader writes to the stack pointer
+  after setting up the stack based on the app header. This means if we
+  set the app stack right at the end of memory then the bootloader
+  hard faults.
+*/
+_estack = 0x20003c00;
 
 /* Generate a link error if heap and stack don't fit into RAM */
 _Min_Heap_Size = 0x200;      /* required amount of heap  */
@@ -34,7 +42,7 @@ FLASH (rx)      : ORIGIN = 0x08001000, LENGTH = 27K
 FLASH_VERSION (rx)  : ORIGIN = 0x08007C00 - 48, LENGTH = 16
 FILE_NAME     (rx)  : ORIGIN = 0x08007C00 - 32, LENGTH = 32
 EEPROM        (rx)  : ORIGIN = 0x08007C00, LENGTH = 1K
-RAM (xrw)       : ORIGIN = 0x20000000, LENGTH = 16K
+RAM (xrw)       : ORIGIN = 0x20000000, LENGTH = 15K
 
 }