boards: shields: esp_8266: Add std headers overlay

The current shield version shows different configuration, including
disabling default ethernet and wifi interfaces.  To allow easy
connection from standard headers, this add two new shield designations.
The existing boards overlays and defconfig were updated.

The documentation was update to reflect changes. A new supported
variations section helps to understand what shield variation user need
select to build their own system.

This changes requires changes on esp32 offload tests to reflect new
esp_8266 shield designations. Sam4e_xpro was keeped as the only one
with dedicated overlay and both frdm_k64f and disco_l475_iot1 were
moved to use standard arduino r3 header using extra args.

Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>