These pins can be used as GPIO's, by default the development board needs some UART TX/RX for its console prints, Since these pins are availbale on the board header they chose these pins for UART,
To use these pins as normal GPIO you should comment out this section in the platform.c file
#ifndef WICED_DISABLE_STDIO /* Initialise UART standard I/O */ platform_stdio_init( &platform_uart_drivers[STDIO_UART], &platform_uart_peripherals[STDIO_UART], &stdio_config ); #endif
Which initializes the Pins as UART pins,
After this you can use these Pins as just any normal GPIO.
I remember there are some issues with using PB* pins as GPIO on 4390.
You had better get confirm from Broadcom.
The only issue is lack of interrupt support. But otherwise they can be used as regular GPIOs