bd: Added get_erase_value function to the block device API
bd: Added get_erase_value function to the block device API

Default implementation returns -1 and is backwards compatible

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