APIs esp_btbb_enable and esp_btbb_disable are supposed to be used by 802.15.4 and Bluetooth module, and implemented with reference counter for resource management
* 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