Add config target.internal-flash-uniform-sectors and set it to false for STM32L2/4/7
Add config target.internal-flash-uniform-sectors and set it to false for STM32L2/4/7

Some internal flashes have non-uniform sectors, and for those
ones we want to skip the initialization of default LittleFileSystem
on FlashIAPBlockDevice (unless the user specifies an address
range that's uniform).

This commit adds a config to indicate if sectors are uniform.

mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed