minimal-printf: Fix handling of the two character sequence %%

The two character sequence %% is used in standard implementations of
printf to print a single %. This is because % is essentially printf's
escape character for format specifiers and as \% cannot work printf
uses %%.
Therefore to be compatible with string buffers containing
%%, minimal-printf also needs to only print a single %.

Author: hugueskamba

platform/source/minimal-printf/mbed_printf_implementation.c

@@ -625,6 +625,9 @@ int mbed_minimal_formatted_string(char *buffer, size_t length, const char *forma
                     index = next_index;
 
                     mbed_minimal_formatted_string_void_pointer(buffer, length, &result, value, stream);
+                } else if (next == '%') {
+                    index = next_index;
+                    mbed_minimal_formatted_string_signed(buffer, length, &result, next, stream);
                 } else {
                     /* write all characters between format beginning and unrecognied modifier */
                     while (index < next_index) {