supervisor: rename some locals for clarity

It's perfectly OK for these variables with static linkage to have the
same name, but it's inconvenient for humans like me.

Author: jepler

supervisor/shared/tick.c

@@ -66,7 +66,7 @@
 
 static volatile uint64_t PLACE_IN_DTCM_BSS(background_ticks);
 
-static background_callback_t callback;
+static background_callback_t tick_callback;
 
 volatile uint64_t last_finished_tick = 0;
 
@@ -119,7 +119,7 @@ void supervisor_tick(void) {
         #endif
     }
 #endif
-    background_callback_add(&callback, supervisor_background_tasks, NULL);
+    background_callback_add(&tick_callback, supervisor_background_tasks, NULL);
 }
 
 uint64_t supervisor_ticks_ms64() {

supervisor/shared/usb/usb.c

@@ -64,8 +64,8 @@ void usb_init(void) {
     tusb_init();
 
 #if MICROPY_KBD_EXCEPTION
-    // Set Ctrl+C as wanted char, tud_cdc_rx_wanted_cb() callback will be invoked when Ctrl+C is received
-    // This callback always got invoked regardless of mp_interrupt_char value since we only set it once here
+    // Set Ctrl+C as wanted char, tud_cdc_rx_wanted_cb() usb_callback will be invoked when Ctrl+C is received
+    // This usb_callback always got invoked regardless of mp_interrupt_char value since we only set it once here
     tud_cdc_set_wanted_char(CHAR_CTRL_C);
 #endif
 
@@ -83,14 +83,14 @@ void usb_background(void) {
     }
 }
 
-static background_callback_t callback;
+static background_callback_t usb_callback;
 static void usb_background_do(void* unused) {
     usb_background();
 }
 
 void usb_irq_handler(void) {
     tud_int_handler(0);
-    background_callback_add(&callback, usb_background_do, NULL);
+    background_callback_add(&usb_callback, usb_background_do, NULL);
 }
 
 //--------------------------------------------------------------------+