Flush serial in DeepSleep example to allow print before sleep

[lbernstone]

Fixes #1790

[stickbreaker]

Does Serial.flush() work now?

[entropia1ac]

not work -no going to sleep. (Board: TTGO LORA32 ,stable release 1.0.0)
........
void setup(){
Serial.begin(115200);
delay(1000); //Take some time to open up the Serial Monitor
++bootCount;
Serial.println("Boot number: " + String(bootCount));
print_wakeup_reason();

Serial.flush();
delay(1000);
esp_sleep_enable_timer_wakeup(TIME_TO_SLEEP * uS_TO_S_FACTOR);
Serial.println("Setup ESP32 to sleep for every " + String(TIME_TO_SLEEP) +
" Seconds");
}
void loop(){
Serial.println("Boot number in loop=" + String(bootCount));
delay(1000);
}
.....
debug:==============
ets Jun 8 2016 00:22:57

rst:0x1 (POWERON_RESET),boot:0x17 (SPI_FAST_FLASH_BOOT)
configsip: 0, SPIWP:0xee
clk_drv:0x00,q_drv:0x00,d_drv:0x00,cs0_drv:0x00,hd_drv:0x00,wp_drv:0x00
mode:DIO, clock div:1
load:0x3fff0018,len:4
load:0x3fff001c,len:808
load:0x40078000,len:6084
load:0x40080000,len:6696
entry 0x400802e4
Boot number: 1
Wakeup was not caused by deep sleep
Setup ESP32 to sleep for every 5 Seconds
Boot number in loop=1
Boot number in loop=1
Boot number in loop=1
Boot number in loop=1
Boot number in loop=1
Boot number in loop=1
Boot number in loop=1
Boot number in loop=1
Boot number in loop=1

[lbernstone]

This is not really the place for issues. The code works fine for me, and you have clearly changed it. Download the actual code (https://github.com/lbernstone/arduino-esp32/blob/sleepsketchfix/libraries/ESP32/examples/DeepSleep/TimerWakeUp/TimerWakeUp.ino) and test that.