Explicitly set the baud rate for the shared Serial

This helps avoiding confusion when different targets have different baud
rate expectation. If we enable the EFM32GG_STK3700 target, for example,
it would fail to print if the baud rate was 9600, the default mbed baud
rate.

Author: AlessandroA

source/main-hw.h

@@ -31,6 +31,7 @@ extern Serial shared_pc;
 #define LED_OFF            true
 #define SECURE_SWITCH      SW2
 #define SECURE_SWITCH_PULL PullUp
+#define SHARED_SERIAL_BAUD 9600
 
 #define MAIN_ACL(acl_list_name) \
     static const UvisorBoxAclItem acl_list_name[] = {     \
@@ -52,7 +53,6 @@ extern Serial shared_pc;
     }
 
 #else /* Target-specific settings */
-
 #error "Unsupported target. Checkout the README.md file for the list of supported targets for this app."
 
 #endif /* Target-specific settings */

source/main.cpp

@@ -38,7 +38,7 @@ DigitalOut led_red(LED1);
 DigitalOut led_green(LED2);
 DigitalOut led_blue(LED3);
 
-Serial shared_pc(USBTX, USBRX);
+Serial shared_pc(USBTX, USBRX, SHARED_SERIAL_BAUD);
 
 static uint32_t get_a_number()
 {