code cleanup 2: explicit .bss, linker map, and simpler systick

SConscript.boardloader

@@ -104,7 +104,7 @@ env.Replace(
     '-fstack-protector-all -ffreestanding '
     + CCFLAGS_MOD,
     CCFLAGS_QSTR='-DNO_QSTR -DN_X64 -DN_X86 -DN_THUMB',
-    LINKFLAGS='-nostdlib -T embed/boardloader/memory.ld --gc-sections',
+    LINKFLAGS='-nostdlib -T embed/boardloader/memory.ld --gc-sections -Map=build/boardloader/boardloader.map',
     CPPPATH=[
         'embed/boardloader',
         'embed/trezorhal',

embed/trezorhal/common.c

@@ -30,7 +30,7 @@ void __attribute__((noreturn)) __fatal_error(const char *expr, const char *msg,
     for (;;);
 }
 
-uint32_t __stack_chk_guard;
+uint32_t __stack_chk_guard = 0;
 
 void __attribute__((noreturn)) __stack_chk_fail(void)
 {

embed/trezorhal/stm32.c

@@ -41,13 +41,9 @@ void SystemInit(void)
     SCB->CPACR |= ((3U << 22) | (3U << 20));
 }
 
+volatile uint32_t uwTick = 0;
+
 void SysTick_Handler(void)
 {
-    extern volatile uint32_t uwTick;
     uwTick += 1;
-    // TODO: verify the following claim, or remove
-    // Read the systick control regster. This has the side effect of clearing
-    // the COUNTFLAG bit, which makes the logic in sys_tick_get_microseconds
-    // work properly.
-    SysTick->CTRL;
 }