sencha: track hardware memory map

The sencha hardware spec has .5M of SRAM at 0x50000000; change the
board spec and take a guess at what heap+shadow should be.

Bypass-Presubmit-Reason: tested as part of the topic

Change-Id: Iea5ec719deccac9af65ea3025339b77826395508

sdk/boards/sencha.json

diff --git a/sdk/boards/sencha.json b/sdk/boards/sencha.json
index 089fc17..92912ce 100644
--- a/sdk/boards/sencha.json
+++ b/sdk/boards/sencha.json
@@ -29,17 +29,17 @@
             "length": 0x40
         },
         "shadow" : {
-            "start": 0x83000000,
-            "end":   0x83010000
+            "start": 0x50077000,
+            "end":   0x50078000
         }
     },
     "instruction_memory": {
-        "start": 0x80000000,
-        "end": 0x80400000
+        "start": 0x50000000,
+        "end": 0x50080000
     },
     "heap": {
-        "start": 0x80380000,
-        "end":   0x80400000
+        "start": 0x50078000,
+        "end":   0x50080000
     },
     "interrupts": [
         {