start adding smoltcp/ethernet support

memory.x

@@ -1,8 +1,23 @@
 
 MEMORY
 {
-  /* Zedboard: 512 MB DDR3. Only use 256 MB for now, should be plenty for a bare-metal app. */
-  CODE(rx) : ORIGIN = 0x00100000, LENGTH = 256M
+  /* Zedboard: 512 MB DDR3. Only use 63 MB for now, should be plenty for a bare-metal app.
+  Leave 1 MB of memory which will be configured as uncached device memory by the MPU. This is
+  recommended for something like DMA descriptors. */
+  CODE(rx) : ORIGIN = 0x00100000, LENGTH = 63M
+  UNCACHED(rx): ORIGIN = 0x4000000, LENGTH = 1M
 }
 
 REGION_ALIAS("DATA", CODE);
+
+SECTIONS
+{
+  /* Uncached memory */
+  uncached (NOLOAD) : ALIGN(4) {
+    . = ALIGN(4);
+    _sbss_uncached = .;
+    *(.bss.uncached .bss.uncached.*);
+    . = ALIGN(4);
+    _ebss_uncached = .;
+  } > UNCACHED
+}