susbys: logging: Added logging to flash backend

New backend is based on littleFS. After init, there is created new file
with continuous numeration. When max size of file is reached, system
creates another one. File size is limited by Kconfig option.
There is possibility to overwrite old files or drop new ones.
FS backend logging to file if the FS location is available.
Otherwise log messages are drooped.

User can also change the name of log files.

Signed-off-by: Mateusz Syc <Mateusz.Syc@nordicsemi.no>
Signed-off-by: Andrzej Puzdrowski <Andrzej.Puzdrowski@nordicsemi.no>

subsys/logging/Kconfig.processing

diff --git a/subsys/logging/Kconfig.processing b/subsys/logging/Kconfig.processing
index bf5a36e..162e096 100644
--- a/subsys/logging/Kconfig.processing
+++ b/subsys/logging/Kconfig.processing
@@ -81,6 +81,7 @@
 	default 4096 if ARM64
 	default 4096 if SPARC
 	default 2048 if (RISCV && 64BIT)
+	default 2048 if LOG_BACKEND_FS
 	default 768
 	help
 	  Set the internal stack size for log processing thread.