Add some limited JSON Patch support for board description files.

Boards can now be provided as .patch files that are JSON documents
containing two nodes:

 - `base` is the board that this is based on, which can be another
   .patch file, a full path (note: relative paths are not yet supported,
   but they should be for out-of-tree BSPs).
 - `patch` is a JSON Patch.

The patch is a subset of RFC 6902.  Current limitations:

 - It doesn't handle escapes in keys or any weird keys.
 - It assumes that numbers in the JSON Pointer path are array indexes
   and will do the wrong thing if you try to replace or add an object
   field with a number as a key.
 - It doesn't implement the copy, move, or test operations, only add,
   remove, and replace.

This is sufficient to be useful.  The Arty A7 and Sonata Simulator JSON
files are now replaced with patches.

sdk/boards/ibex-arty-a7-100.patch

@@ -0,0 +1,77 @@
+{
+  "base": "ibex-safe-simulator",
+  "patch": [
+    {
+      "op": "add",
+      "path": "/devices/gpio_led0",
+      "value": {
+        "start": 0x8f00f000,
+        "length": 0x800
+      }
+    },
+    {
+      "op": "add",
+      "path": "/kunyan_ethernet",
+      "value": {
+        "start": 0x14004000,
+        "end": 0x14008000
+      }
+    },
+    {
+      "op": "add",
+      "path": "/interrupts/1",
+      "value": {
+        "name": "UARTInterrupt",
+        "number": 2,
+        "priority": 3,
+        "edge_triggered": true
+      }
+    },
+    {
+      "op": "add",
+      "path": "/interrupts/2",
+      "value": {
+        "name": "EthernetTransmitInterrupt",
+        "number": 3,
+        "priority": 3
+      }
+    },
+    {
+      "op": "add",
+      "path": "/interrupts/3",
+      "value": {
+        "name": "EthernetReceiveInterrupt",
+        "number": 4,
+        "priority": 3
+      }
+    },
+    {
+      "op": "add",
+      "path": "/driver_includes/0",
+      "value": "${sdk}/include/platform/synopsis"
+    },
+    {
+      "op": "add",
+      "path": "/driver_includes/0",
+      "value": "${sdk}/include/platform/arty-a7"
+    },
+    {
+      "op": "replace",
+      "path": "/timer_hz",
+      "value": 33000000
+    },
+    {
+      "op": "replace",
+      "path": "/tickrate_hz",
+      "value": 100
+    },
+    {
+      "op": "remove",
+      "path": "/simulation"
+    },
+    {
+      "op": "remove",
+      "path": "/simulator"
+    }
+  ]
+}

sdk/boards/sonata-simulator.patch

@@ -0,0 +1,15 @@
+{
+  "base": "sonata-prerelease",
+  "patch": [
+    {
+      "op": "replace",
+      "path": "/simulation",
+      "value": true
+    },
+    {
+      "op": "replace",
+      "path": "/simulator",
+      "value": "${sdk}/../scripts/run-sonata-sim.sh"
+    }
+  ]
+}