Fix dependency tree so that lwip doesn't depend on any specific network
interface component.
Network interface drivers shall depend on esp_netif.
esp_netif shall depend on lwip (but not on any specific interface
driver) -- it optionally depends on vfs and esp_eth (need ethernet
header for L2/bridge mode)
* add esp-phy-lib submodule
* move libphy.a and phy_init.c from esp_wifi to esp_phy
* move librtc.a from esp_wifi to esp_phy
* move libbtbb.a from bt to esp_phy
* corresponding updates to build system
