PPP is now enabled on all boards with Ethernet support. PPP could be
enabled for other boards without Ethernet like the Teensy 4.0 as well in a
second step. Enabling for MIMXRT101x boards is hardly possible due to the
large RAM demand of lwIP.
Tested with a Teensy 4.1 board and a SimCom A7608 GPRS/LTE modem.
Signed-off-by: robert-hh <robert@hammelrath.com>