Use umm_malloc for heap management

tools/sdk/lib/README.md

@@ -0,0 +1,13 @@
+## Updating SDK libraries
+
+After updating SDK libraries to a new version, do the following changes.
+
+
+1. Weaken symbols from mem_manager.o to use custom heap implementation
+
+    ```bash
+    xtensa-lx106-elf-ar -x libmain.a mem_manager.o
+    xtensa-lx106-elf-objcopy --weaken mem_manager.o
+    xtensa-lx106-elf-ar -r libmain.a mem_manager.o
+    rm mem_manager.o
+    ```