lisa._assets.kmodules.lisa: Create VFS for lisa module

In order to configure lisa features and their parameters at
runtime, add support for a VFS in the lisa module.

The lisa VFS can be mounted as:
$ mkdir /lisa
$ mount -t lisa none /lisa

It contains the following files/folders:
lisa_fs
\-activate                  # Activate this config
\-available_features        # List of available features
\-event__lisa__perf_counter # (E.g.) Dir to configure perf_counter feature
  \-generic_counters        # Generic counter to track
  \-pmu_raw_counters        # PMU raw counters to track
\-configs                   # Allow multiple configurations
  \-custom_conf_0           # Custom configuration ()
    \- ...                  # Similar files as in lisa_fs/
\-set_features              # Features to enable

A feature can have various parameters. For instance, the
'event__lisa__perf_counter' feature has two parameters that can
be configured: 'generic_counters', 'pmu_raw_counters'.

Multiple configurations can co-exist. The 'main' configuration is shown
above, and other custom configurations can be created in the 'configs'
directory.
If multiple configurations are active simultaneously, the VFS merges
them into one global configuration. For instance, if:
- feat_0 is set in custom_conf_0
- feat_1 is set in custom_conf_1
and both configurations are active, the resulting configuration will
have feat_0 and feat_1 active.

Original-author: Beata Michalska <beata.michalska@arm.com>
Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>