flow: fs node transform into persistence node - with efivars

fs node renamed to persistence node. As it will not only deal with fs
anymore. Besides it, a new option was added: 'storage', which defines
storage destination, currently 'fs' and 'efivars'. When using 'fs',
'name' can be a fully fledged path to the file where value will be
persisted.
'efivars' saves on 'efivarfs', using a reasonable attributes set:
"EFI_VARIABLE_NON_VOLATILE | EFI_VARIABLE_BOOTSERVICE_ACCESS |
EFI_VARIABLE_RUNTIME_ACCESS".
'fs' now creates files in case they don't happen to exist.
When there is no previous persisted value, node 'default_value' will
be used instead.

Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>

src/lib/flow/Kconfig

@@ -45,7 +45,6 @@ source "src/modules/flow/file/Kconfig"
 source "src/modules/flow/filter-repeated/Kconfig"
 source "src/modules/flow/float/Kconfig"
 source "src/modules/flow/freegeoip/Kconfig"
-source "src/modules/flow/fs/Kconfig"
 source "src/modules/flow/gpio/Kconfig"
 source "src/modules/flow/grove/Kconfig"
 source "src/modules/flow/gtk/Kconfig"
@@ -58,6 +57,7 @@ source "src/modules/flow/magnetometer/Kconfig"
 source "src/modules/flow/max31855/Kconfig"
 source "src/modules/flow/network/Kconfig"
 source "src/modules/flow/oic/Kconfig"
+source "src/modules/flow/persistence/Kconfig"
 source "src/modules/flow/piezo-speaker/Kconfig"
 source "src/modules/flow/platform/Kconfig"
 source "src/modules/flow/process/Kconfig"