shell: Allow extension of shell APIs

Similar to logging module, allow application specific extension of
shell fprintf APIs at a macro level.

Signed-off-by: Al Semjonovs <asemjonovs@google.com>

include/zephyr/shell/shell.h

@@ -1008,9 +1008,10 @@ int shell_stop(const struct shell *sh);
  * @param[in] fmt	Format string.
  * @param[in] ...	List of parameters to print.
  */
-void __printf_like(3, 4) shell_fprintf(const struct shell *sh,
-				       enum shell_vt100_color color,
-				       const char *fmt, ...);
+void __printf_like(3, 4) shell_fprintf_impl(const struct shell *sh, enum shell_vt100_color color,
+					    const char *fmt, ...);
+
+#define shell_fprintf(sh, color, fmt, ...) shell_fprintf_impl(sh, color, fmt, ##__VA_ARGS__)
 
 /**
  * @brief vprintf-like function which sends formatted data stream to the shell.
@@ -1277,4 +1278,9 @@ int shell_get_return_value(const struct shell *sh);
 }
 #endif
 
+#ifdef CONFIG_SHELL_CUSTOM_HEADER
+/* This include must always be at the end of shell.h */
+#include <zephyr_custom_shell.h>
+#endif
+
 #endif /* SHELL_H__ */

subsys/shell/Kconfig

@@ -297,6 +297,14 @@ config SHELL_CMDS_RETURN_VALUE
 	  This option enables the retval command. It is used to retrieve
 	  the return value from the most recently executed command.
 
+config SHELL_CUSTOM_HEADER
+	bool "Include Custom Shell Header"
+	help
+	  When enabled, a custom application provided header, named
+	  "zephyr_custom_shell.h", is included at the end of shell.h. This enables
+	  extension of the shell APIs at the macro level. Please use cautiously!
+	  The internal shell API may change in future releases.
+
 source "subsys/shell/modules/Kconfig"
 
 endif # SHELL

subsys/shell/shell.c

@@ -1548,7 +1548,7 @@ void shell_vfprintf(const struct shell *sh, enum shell_vt100_color color,
 /* This function mustn't be used from shell context to avoid deadlock.
  * However it can be used in shell command handlers.
  */
-void shell_fprintf(const struct shell *sh, enum shell_vt100_color color,
+void shell_fprintf_impl(const struct shell *sh, enum shell_vt100_color color,
 		   const char *fmt, ...)
 {
 	va_list args;