Add support of heap memory split between 2-RAM banks.

Please note the heap address of the both the banks must not be contigious else
GCC considers it to be single memory bank and does allocation across the banks,
which might result into hard-fault

Author: Deepika

platform/mbed_retarget.cpp

@@ -1240,6 +1240,41 @@ extern "C" WEAK void __cxa_pure_virtual(void)
 // SP.  This make it compatible with RTX RTOS thread stacks.
 #if defined(TOOLCHAIN_GCC_ARM)
 
+#if defined(MBED_SPLIT_HEAP)
+
+extern uint32_t __mbed_sbrk_start;
+extern uint32_t __mbed_krbs_start;
+extern uint32_t __mbed_sbrk_start_0;
+extern uint32_t __mbed_krbs_start_0;
+
+extern "C" WEAK caddr_t _sbrk(int incr)
+{
+    static uint32_t heap = (uint32_t) &__mbed_sbrk_start_0;
+    static bool once = true;
+    uint32_t prev_heap = heap;
+    uint32_t new_heap = heap + incr;
+
+    /**
+     * If the new address is outside the first region, start allocating from the second region.
+     */
+    if (once && (new_heap > (uint32_t) &__mbed_krbs_start_0)) {
+        once = false;
+        prev_heap = (uint32_t) &__mbed_sbrk_start;
+        new_heap = prev_heap + incr;
+    } else if (new_heap > (uint32_t) &__mbed_krbs_start) {
+        /**
+        * If the new address is outside the second region, return out-of-memory.
+        */
+        errno = ENOMEM;
+        return (caddr_t) - 1;
+    }
+
+    heap = new_heap;
+    return (caddr_t) prev_heap;
+}
+
+#else
+
 extern "C" uint32_t         __end__;
 extern "C" uint32_t         __HeapLimit;
 
@@ -1264,6 +1299,7 @@ extern "C" WEAK caddr_t _sbrk(int incr)
     return (caddr_t) prev_heap;
 }
 #endif
+#endif
 
 #if defined(TOOLCHAIN_GCC_ARM)
 extern "C" void _exit(int return_code)

targets/TARGET_STM/TARGET_STM32L4/l4_retarget.c

@@ -4147,7 +4147,8 @@
         "detect_code": ["0764"],
         "macros_add": [
             "MBED_TICKLESS",
-            "USBHOST_OTHER"
+            "USBHOST_OTHER",
+            "MBED_SPLIT_HEAP"
         ],
         "device_has_add": [
             "ANALOGOUT",
@@ -4178,7 +4179,7 @@
             }
         },
         "detect_code": ["0468"],
-        "macros_add": ["USBHOST_OTHER", "TWO_RAM_REGIONS"],
+        "macros_add": ["USBHOST_OTHER", "MBED_SPLIT_HEAP"],
         "device_has_add": [
             "ANALOGOUT",
             "CAN",