Esp8266 and Esp32 PHY init data storage

[mikee47]

PHY init data is only 128 bytes so using an entire sector is not optimal. We could refactor to include the data as a BLOB in each ROM.

esp-idf does exactly this - see https://docs.espressif.com/projects/esp-idf/en/latest/esp32/api-guides/partition-tables.html,
although the CONFIG_ESP_PHY_INIT_DATA_IN_PARTITION can be enabled to use a partition.

Advantages:

• Saves a flash sector
• Simplifies the map since we can remove phy_init

Disadvantages:

• Some existing applications may use customised flash data partitions
• SDK may require a phy_init partition at boot time, though that can probably be fudged

[mikee47]

See also https://sming.readthedocs.io/en/latest/information/tips-n-tricks.html#reading-vcc-on-esp8266 which would be affected.