test_fs: Allow testing FS_GENERIC

test_fs: Allow testing FS_GENERIC

The generic filesystem interface was so far untested. The interface
is similar to the FS specific interfaces with FS specific prefixes,
like ext4ls, fatmkdir, ... but it does not have any prefixes, i.e.
it provides plain ls, mkdir, ... commands.

Extend the test parameters to include 'fs_cmd_prefix' and optionally
'fs_cmd_write' parameters. The 'fs_cmd_prefix' allow specifying the
filesystem specific command prefix, like 'ext4' in 'ext4ls'. The
'fs_cmd_write' allows selecting between 'write'/'save' command name
for storing files into the filesystem, see last paragraph.

Introduce new 'fs_generic' fs_type which is used to parametrize existing
tests and run them without any prefixes if detected, thus testing the
generic filesystem interface. Use the fatfs as the backing store for the
generic FS tests.

The check_ubconfig needs to be slightly adjusted to avoid test for
CMD_FS_GENERIC_WRITE which does not exist separately from CMD_FS_GENERIC.

The CMD_FS_GENERIC does not provide generic 'write' command, instead
the generic equivalent command is called 'save' . Add simple ternary
oeprator to use 'save' command for CMD_FS_GENERIC tests and '..write'
commands for filesystem specific tests.

Enable generic filesystem tests for basic/extended/mkdir/unlink tests.

Signed-off-by: Marek Vasut <marex@denx.de>