Link against newlib from esp-open-sdk's libcirom, replace FreeRTOS heap management.

Compiles and runs, expects xtensa lock.c patch in newlib (will still run
otherwise but malloc/free and other functions will be non-memory-safe...)

core/newlib_syscalls.c

@@ -0,0 +1,71 @@
+/* newlib_syscalls.c - newlib syscalls for ESP8266
+ *
+ * Part of esp-open-rtos
+ * Copyright (C) 2105 Superhouse Automation Pty Ltd
+ * BSD Licensed as described in the file LICENSE
+ */
+#include <sys/reent.h>
+#include <sys/types.h>
+#include <espressif/sdk_private.h>
+#include <stdlib.h>
+
+caddr_t _sbrk_r (struct _reent *r, int incr)
+{
+    extern char   _heap_start; /* linker script defined */
+    static char * heap_end;
+    char *        prev_heap_end;
+
+    if (heap_end == NULL)
+	heap_end = &_heap_start;
+    prev_heap_end = heap_end;
+    /* TODO: Check stack collision
+       if (heap_end + incr > stack_ptr)
+       {
+       _write (1, "_sbrk: Heap collided with stack\n", 32);
+       while(1) {}
+       }
+    */
+    heap_end += incr;
+
+    return (caddr_t) prev_heap_end;
+}
+
+/* syscall implementation for stdio write to UART
+
+   at the moment UART functionality is all still in the binary SDK
+ */
+long _write_r(struct _reent *r, int fd, const char *ptr, int len )
+{
+    for(int i = 0; i < len; i++)
+	sdk_os_putc(ptr[i]);
+    return len;
+}
+
+/* syscall implementation for stdio read from UART
+
+   at the moment UART functionality is all still in the binary SDK
+ */
+long _read_r( struct _reent *r, int fd, char *ptr, int len )
+{
+    for(int i = 0; i < len; i++)
+	ptr[i] = sdk_uart_rx_one_char();
+    return len;
+}
+
+/* These are stub implementations for the reentrant syscalls that
+ * newlib is configured to expect */
+int _fstat_r(struct _reent *r, int fd, void *buf)
+{
+    return -1;
+}
+
+int _close_r(struct _reent *r, int fd)
+{
+    return -1;
+}
+
+off_t _lseek_r(struct _reent *r, int fd, off_t offset, int whence)
+{
+    return (off_t)-1;
+}
+