Add menuconfig support for setting device hostname and device WIFI MAC address as serial

[hire-vladimir]

Three enhancements:

• Allow menuconfig setting of device hostname vs original espressif (new default is esp-cam)
• Allow menuconfig support for setting device serial number to WIFI MAC address, still keeping recently added option for custom string
  • As inspired by idea mentioned in #14
• Also one minor tweak to README.md to clarify how to interact with menuconfig, per comments in #60

Sorry @maximkulkin i did not know on your plans on doing the earlier mentioned variable name refactoring, these will need adjustment also.

On side note as i was working to update the device hostname, some of the wifi methods have been deprecated with IDF 4.x and will need to be updated to new example per https://github.com/espressif/esp-idf/blob/master/examples/wifi/getting_started/station/main/station_example_main.c, i've opened #71 to keep track.

esp_event_loop.h:2:2: warning: #warning "esp_event_loop.h is deprecated, please include esp_event.h instead" [-Wcpp]
app_main.c:49:5: warning: 'tcpip_adapter_init' is deprecated [-Wdeprecated-declarations]
app_main.c:51:5: warning: 'esp_event_loop_init' is deprecated [-Wdeprecated-declarations]