subsys/fs: Add open flags parameter to fs_open

The fs_open has been extended with support for open flags.
Currently supported flags are:
  FS_O_READ -- open for read
  FS_O_WRITE -- open for write
  FS_O_CREATE -- create file if it does not exist
  FS_O_APPEND -- move to the end of file before each write

The FAT FS and LittleFS front-ends within the Zephyr has also been
modified to utilize the flags.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>