Support ESP-IDF Install from External Directory

[oclyke]

Hi there.

Thanks a bunch for maintaining this library.

This PR makes it easy to include the library when it is not directly within the components directory by simply setting the ESP_DMX_ROOT variable before including the CMakeLists.txt file.

This allows users to create a simple wrapper component like this:

# components/esp_dmx_wrapper/CMakeLists.txt
set(ESP_DMX_ROOT <path to esp_dmx>)
include("${ESP_DMX_ROOT}/CMakeLists.txt")

The benefit, to me, is that I can keep all my third-party dependencies / submodules in one place. It makes it easier not to miss one ;)

What do you think?