shell: Allow custom shell log backend implementation

Shell is capable of acting as the logging backend and multiplex
logging messages with shell content. It expects that logging
messages goes through the logging backend. However, there is an
option that logs go only to the custom logging frontend. In
that case this default approach does not work and logging
frontend cannot be easily integrated with the shell (due to
its custom nature). Add Kconfig option SHELL_LOG_BACKEND_OOB
which prevent compilation of the default shell log backend.
When enabled, application can provide frontend specific
implementation to integrate shell with the logging.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>

subsys/shell/CMakeLists.txt

@@ -11,10 +11,12 @@ zephyr_sources_ifdef(
   shell_ops.c
   )
 
-zephyr_sources_ifdef(
-  CONFIG_SHELL_LOG_BACKEND
-  shell_log_backend.c
+if(NOT CONFIG_SHELL_LOG_BACKEND_CUSTOM)
+  zephyr_sources_ifdef(
+    CONFIG_SHELL_LOG_BACKEND
+    shell_log_backend.c
   )
+endif()
 
 zephyr_sources_ifdef(
   CONFIG_SHELL_HELP

subsys/shell/Kconfig

@@ -277,6 +277,13 @@ config SHELL_LOG_BACKEND
 	  using the shell backend's LOG_LEVEL option
 	  (e.g. CONFIG_SHELL_TELNET_INIT_LOG_LEVEL_NONE=y).
 
+config SHELL_LOG_BACKEND_CUSTOM
+	bool "Use custom backend implementation"
+	help
+	  When enabled, standard implementation of shell log backend is not included.
+	  It allows to provide custom implementation of multiplexing logging messages
+	  with shell data.
+
 config SHELL_LOG_FORMAT_TIMESTAMP
 	bool "Format timestamp"
 	default y