Remove Yotta support from the docs, tests and build scripts

Yotta is no longer supported by Mbed TLS, so has been removed. Specifically, the
following changes have been made:
 * references to yotta have been removed from the main readme and build
   instructions
 * the yotta module directory and build script has been removed
 * yotta has been removed from test scripts such as all.sh and check-names.sh
 * yotta has been removed from other files that that referenced it such as the
   doxyfile and the bn_mul.h header
 * yotta specific configurations and references have been removed from config.h
diff --git a/include/mbedtls/bn_mul.h b/include/mbedtls/bn_mul.h
index b587317..6679389 100644
--- a/include/mbedtls/bn_mul.h
+++ b/include/mbedtls/bn_mul.h
@@ -565,9 +565,8 @@
 #endif /* TriCore */
 
 /*
- * gcc -O0 by default uses r7 for the frame pointer, so it complains about our
- * use of r7 below, unless -fomit-frame-pointer is passed. Unfortunately,
- * passing that option is not easy when building with yotta.
+ * Note, gcc -O0 by default uses r7 for the frame pointer, so it complains about
+ * our use of r7 below, unless -fomit-frame-pointer is passed.
  *
  * On the other hand, -fomit-frame-pointer is implied by any -Ox options with
  * x !=0, which we can detect using __OPTIMIZE__ (which is also defined by