littlefs: Update HAL flash API usage

MCU-Driver-HAL's flash API has been updated to a new programming model
based on a vtable. The instantiation and initialization of flash
are now platform-specific, but once it's done subsequent API calls on
the vtable object are fully generic. This commit updates the flash API
usage in the littlefs example.

To allow the littlefs example to potentially be built and run on
different platforms, we separate the main application (`main.c`) and
Corstone-300 flash initialization (`platforms/flash_cs300.c`).

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>