Put persistent variables at end of stack

Two variables of the bootloader are persistent:
- __stack_chk_guard is used by interrupt handlers
- system_millis is used by timer interrupt and service routines

Note that currently __stack_chk_guard is shared between unprivileged
firmware and bootloader.

If we get more variables later it may make sense to make a section for
this.

memory.ld

@@ -18,5 +18,8 @@ INCLUDE libopencm3_stm32f2.ld
 
 _ram_start = ORIGIN(ram);
 _ram_end = ORIGIN(ram) + LENGTH(ram);
+_stack = _ram_end - 8;
+__stack_chk_guard = _ram_end - 8;
+system_millis = _ram_end - 4;
 
 _data_size = SIZEOF(.data);

memory_app_1.0.0.ld

@@ -18,5 +18,8 @@ INCLUDE libopencm3_stm32f2.ld
 
 _ram_start = ORIGIN(ram);
 _ram_end = ORIGIN(ram) + LENGTH(ram);
+_stack = _ram_end - 8;
+__stack_chk_guard = _ram_end - 8;
+system_millis = _ram_end - 4;
 
 _data_size = SIZEOF(.data);

setup.c

@@ -29,8 +29,6 @@
 #include "layout.h"
 #include "util.h"
 
-uint32_t __stack_chk_guard;
-
 static inline void __attribute__((noreturn)) fault_handler(const char *line1) {
 	layoutDialog(&bmp_icon_error, NULL, NULL, NULL, line1, "detected.", NULL, "Please unplug", "the device.", NULL);
 	for (;;) {} // loop forever

timer.c

@@ -25,7 +25,7 @@
 #include <libopencm3/cm3/vector.h>
 
 /* 1 tick = 1 ms */
-volatile uint32_t system_millis;
+extern volatile uint32_t system_millis;
 
 /*
  * Initialise the Cortex-M3 SysTick timer

util.h

@@ -48,6 +48,7 @@ extern void __attribute__((noreturn)) shutdown(void);
 #if !EMULATOR
 // defined in memory.ld
 extern uint8_t _ram_start[], _ram_end[];
+extern uint8_t _stack[];
 
 // defined in startup.s
 extern void memset_reg(void *start, void *stop, uint32_t val);
@@ -59,13 +60,13 @@ static inline void __attribute__((noreturn)) jump_to_firmware(const vector_table
 {
 	if (FW_SIGNED == trust) {                 // trusted signed firmware
 		SCB_VTOR = (uint32_t)vector_table;    // * relocate vector table
+		// Set stack pointer
+		__asm__ volatile("msr msp, %0" :: "r" (vector_table->initial_sp_value));
 	} else {                                  // untrusted firmware
 		mpu_config();                         // * configure MPU
+		__asm__ volatile("msr msp, %0" :: "r" (_stack));
 	}
 
-	// Set stack pointer
-	__asm__ volatile("msr msp, %0" :: "r" (vector_table->initial_sp_value));
-
 	// Jump to address
 	vector_table->reset();