partition_table: Support same fallback logic as bootloader for default boot partition

Generates correct "make flash" command even when partition table has no factory partition.

Also adds unit tests for parttool.py

Closes https://github.com/espressif/esp-idf/issues/2086

components/partition_table/gen_esp32part.py

@@ -106,6 +106,38 @@ class PartitionTable(list):
         else:
             return super(PartitionTable, self).__getitem__(item)
 
+    def find_by_type(self, ptype, subtype):
+        """ Return a partition by type & subtype, returns
+        None if not found """
+        TYPES = PartitionDefinition.TYPES
+        SUBTYPES = PartitionDefinition.SUBTYPES
+        # convert ptype & subtypes names (if supplied this way) to integer values
+        try:
+            ptype = TYPES[ptype]
+        except KeyError:
+            try:
+                ptypes = int(ptype, 0)
+            except TypeError:
+                pass
+        try:
+            subtype = SUBTYPES[int(ptype)][subtype]
+        except KeyError:
+            try:
+                ptypes = int(ptype, 0)
+            except TypeError:
+                pass
+
+        for p in self:
+            if p.type == ptype and p.subtype == subtype:
+                return p
+        return None
+
+    def find_by_name(self, name):
+        for p in self:
+            if p.name == name:
+                return p
+        return None
+
     def verify(self):
         # verify each partition individually
         for p in self:
@@ -202,7 +234,7 @@ class PartitionDefinition(object):
         "encrypted" : 0
     }
 
-    # add subtypes for the 16 OTA slot values ("ota_XXX, etc.")
+    # add subtypes for the 16 OTA slot values ("ota_XX, etc.")
     for ota_slot in range(16):
         SUBTYPES[TYPES["app"]]["ota_%d" % ota_slot] = 0x10 + ota_slot