littlefs example: Add missing elf to bin conversion

The linkers' output format of executables is ELF which contains useful
information such as section addresses and debug symbols. But sometimes
raw binaries are needed, for example when a board's disk mount takes a
raw binary image for flashing.

We always generate a raw binary for each example so users and CTest can
use it as needed. This was missing for the littlefs example.

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