logging: Fix false strdup buffer freeing

Algorithm for freeing strdup buffers was only checking if argument
matches address within strdup buffer pool and was attempting freeing
even if format specifier was different than string.

Added fix where also format specifier is checked.

Extended logger test to verify correctness of function which searches
for string format specifiers within a string.

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

include/logging/log_core.h

@@ -630,6 +630,18 @@ void log_dropped(void);
 void __printf_like(2, 3) log_from_user(struct log_msg_ids src_level,
 				       const char *fmt, ...);
 
+/**
+ * @brief Create mask with occurences of a string format specifiers (%s).
+ *
+ * Result is stored as the mask (argument n is n'th bit). Bit is set if string
+ * format specifier was found.
+ *
+ * @param str String.
+ * @param nargs Number of arguments in the string.
+ *
+ * @return Mask with %s format specifiers found.
+ */
+u32_t z_log_get_s_mask(const char *str, u32_t nargs);
 
 /* Internal function used by log_from_user(). */
 __syscall void z_log_string_from_user(u32_t src_level_val, const char *str);