Support dropping named partitions in hardware configs (#2934)

This PR adds the ability to 'drop' a previously defined partition from a hardware configuration by setting `"size": 0` in an inherited config. This is silently ignored if the partition is undefined.

Issue #2933 highlighted the need to be able to eliminate the `factory` partition and use OTA partitions instead. 

A couple of sample applications contained redundant `subtype` overrides since these are already defined in `spiffs-two-roms`, so this has been tidied.

Note: Typically the Esp32 `spiffs-two-roms` configuration is used as a base for OTA updates. This retains the `factory` partition so if not required this can be dropped to liberate the 1M allocation.

Sming/Components/Storage/README.rst

@@ -133,12 +133,18 @@ To customise the hardware configuration for a project, for example 'my_project':
             "rom0": {
                "address": "0x10000",
                "size": "0x80000"
+            },
+            "factory": {
+               "size": 0
             }
          }
       }
 
    This will adjust flash parameters (previously via SPI_SPEED, SPI_MODE and SPI_SIZE),
    and the location/size of the primary application partition.
+   The **factory** partition is also dropped by setting its size to 0.
+   This is ignored if no such partition is defined.
+
 
 3. Add any additional partitions:
 

Sming/Components/Storage/Tools/hwconfig/partition.py

@@ -143,6 +143,11 @@ def parse_dict(self, data, devices):
                 raise InputError("Duplicate partition '%s'" % name)
             partnames += name
             part = self.find_by_name(name)
+            # Setting size=0 drops partition if it exists
+            if entry.get('size') == 0:
+                if part:
+                    self.remove(part)
+                continue
             if part is None:
                 part = Entry(devices[0], name)
                 self.append(part)

Sming/Libraries/OtaUpgradeMqtt/samples/Upgrade/component.mk

@@ -49,7 +49,7 @@ COMPONENT_DEPENDS := OtaUpgradeMqtt
 RBOOT_ENABLED := 1
 
 ## Use standard hardware config with two ROM slots and two SPIFFS partitions
-HWCONFIG := ota
+HWCONFIG := spiffs-two-roms
 
 APP_CFLAGS = -DMQTT_URL="\"$(MQTT_URL)"\"                \
 			 -DMQTT_FINGERPRINT_SHA1=$(MQTT_FINGERPRINT_SHA1)   \

samples/Basic_Ota/ota.hw

@@ -2,12 +2,6 @@
 	"name": "Basic OTA sample",
 	"base_config": "spiffs-two-roms",
 	"partitions": {
-		"rom0": {
-			"subtype": "ota_0"
-		},
-		"rom1": {
-			"subtype": "ota_1"
-		},
 		"spiffs0": {
 			"size": "512K"
 		},