Fix corrupted %f printf because of disabled heap

newlib requires malloc if printf is used with %f. Since PR 28486, the heap
space in RAM is only reserved if explicitly used by the application
(independent of the value of CONFIG_HEAP_MEM_POOL_SIZE) in order to free
some stack memory. As printf calls malloc internally, this commit forces
Zephyr to build with heap support enabled.

src/setup.cpp

@@ -96,6 +96,13 @@ void setup()
     static struct k_timer timestamp_timer;
     k_timer_init(&timestamp_timer, timestamp_inc, NULL);
     k_timer_start(&timestamp_timer, K_MSEC(1000), K_MSEC(1000));
+
+    /*
+     * printf from newlib-nano requires malloc, but Zephyr garbage-collects heap management if it
+     * is not used anywhere in the code. Below dummy calls force Zephyr to build with heap support.
+     */
+    void *temp = k_malloc(4);
+    k_free(temp);
 }
 
 #endif

zephyr/prj.conf

@@ -1,7 +1,7 @@
 # Main application configuration (overrides board-specific settings)
 
-# default is 1024
-CONFIG_HEAP_MEM_POOL_SIZE=1024
+# only very small heap necessary for malloc in printf statements with %f
+CONFIG_HEAP_MEM_POOL_SIZE=256
 
 CONFIG_CPLUSPLUS=y