[Gen4] fixes deadlock in USB hal

[XuGuohui]

Problem

Device may reside in blocked state after reset or waking up from hibernate mode.

1. HAL_USB_Init() in main.cpp sets initialized_ to true and usb thread starts to run with OS_THREAD_PRIORITY_NETWORK - 1 priority
2. USB_USART_LineCoding_BitRate_Handler() in main.cpp or Serial.begin() in app thread with lower priority -> HAL_USB_USART_Begin() -> HAL_USB_Detach() set initialized_ to false
3. usb thread continues to run and keeps waiting for initialized_ to be true in a busy loop
4. app thread is blocked and HAL_USB_Attach() won’t be executed and initialized_ won’t be set true goto #3

Solution

Add a minimum delay within the usb busy loop to allow other threads to call into HAL_USB_Attach()

Steps to Test

Device running the attached app should behave as expected for a long mount of time

Example App

#include "application.h"

SYSTEM_MODE(SEMI_AUTOMATIC);
SYSTEM_THREAD(ENABLED);

SerialLogHandler l(115200, LOG_LEVEL_ALL);

void setup() {
}

void loop() {
    Particle.connect();
    waitFor(Particle.connected, 60000);

    Log.info("Start sleep test");

    SystemSleepConfiguration config;

    config.mode(SystemSleepMode::HIBERNATE)
        .gpio(D8, FALLING)
        .duration(10s);

    Log.info("going to sleep");
    delay(10);

    System.sleep(config);
}

References

N/A

---

Completeness

• [x] User is totes amazing for contributing!
• [x] Contributor has signed CLA (Info here)
• [x] Problem and Solution clearly stated
• [ ] Run unit/integration/application tests on device
• [ ] Added documentation
• [ ] Added to CHANGELOG.md after merging (add links to docs and issues)