STM: Fix us_ticker timestamp after deep sleep

Use the `mbed_sdk_inited` flag to correct the `HAL_GetTick()` behavior
after waking up from deep sleep mode. `ticker_read_us()` must not be
used to read us_ticker timestamp after waking up until the us_ticker
context is restored. More detailed description in issue ARMmbed#8117.

Fixes ARMmbed#8117

Author: Filip Jagodzinski

targets/TARGET_STM/sleep.c

@@ -158,6 +158,7 @@ void hal_sleep(void)
 }
 
 extern int serial_is_tx_ongoing(void);
+extern int mbed_sdk_inited;
 
 void hal_deepsleep(void)
 {
@@ -200,6 +201,10 @@ void hal_deepsleep(void)
     HAL_PWR_EnterSTOPMode(PWR_LOWPOWERREGULATOR_ON, PWR_STOPENTRY_WFI);
 #endif /* TARGET_STM32L4 */
 
+    /* Prevent HAL_GetTick() from using ticker_read_us() to read the
+     * us_ticker timestamp until the us_ticker context is restored. */
+    mbed_sdk_inited = 0;
+
     // Verify Clock Out of Deep Sleep
     ForceClockOutofDeepSleep();
 
@@ -214,6 +219,10 @@ void hal_deepsleep(void)
 
     restore_timer_ctx();
 
+    /* us_ticker context restored, allow HAL_GetTick() to read the us_ticker
+     * timestamp via ticker_read_us() again. */
+    mbed_sdk_inited = 1;
+
     // Enable IRQs
     core_util_critical_section_exit();
 }