esp_modem: Moved to component folder

components/esp_modem/examples/linux_modem/README.md

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+# Linux modem example
+
+This is an experimental port of the esp_modem to linux.
+It mocks some IDF functionality with `port/linux` layers (used for modem host test suite) and implements `esp_netif`,
+which supports `tun` interface and uses lwIP `ppp` implementation to parse or wrap IP packets to be send/receive
+over PPPoS, i.e. over the modem serial line.
+
+## Configuration
+
+* Set path to the lwip and lwip_contrib repositories as environmental variables:
+  - `LWIP_PATH`: path to the lwip repository
+  - `LWIP_CONTRIB_PATH`: path to the lwip_contrib repository
+* Create a `tun` interface using `make_tun_netif` script.
+* Set SIO dev name directly in the code: This is the serial port which is the modem connected to
+* (Set the tun device na interface name in the code: Not needed if the device was created using the script above.)
+* Build and run the example (no elevated privileges needed)
+* Experiment with the network, after getting the IP from the modem device
+    - directly in the code
+    - in the system (need to set `tun` interface IP, dns servers, and routing the desired traffic over the tun interface)