bd: Add sync function to the block device API
bd: Add sync function to the block device API

/** Ensure data on storage is in sync with the driver
 *
 *  @return         0 on success or a negative error code on failure
 */
virtual int sync()

c1728p9/mbed-os,andcor02/mbed-os,betzw/mbed-os,kjbracey-arm/mbed,kjbracey-arm/mbed,andcor02/mbed-os,c1728p9/mbed-os,mbedmicro/mbed,andcor02/mbed-os,c1728p9/mbed-os,andcor02/mbed-os,mbedmicro/mbed,andcor02/mbed-os,betzw/mbed-os,kjbracey-arm/mbed,karsev/mbed-os,karsev/mbed-os,mbedmicro/mbed,mbedmicro/mbed,betzw/mbed-os,betzw/mbed-os,karsev/mbed-os,andcor02/mbed-os,karsev/mbed-os,c1728p9/mbed-os,mbedmicro/mbed,betzw/mbed-os,kjbracey-arm/mbed,karsev/mbed-os,betzw/mbed-os,c1728p9/mbed-os,karsev/mbed-os,c1728p9/mbed-os