This also fixes deleting the PPP task, since eTaskGetState() never returns
eDeleted.
A limitation with this patch: once the PPP is deactivated (ppp.active(0))
it cannot be used again. A new PPP instance must be created instead.
Configuration for the build is now specified using sdkconfig rather than
sdkconfig.h, which allows for much easier configuration with defaults from
the ESP IDF automatically applied. sdkconfig.h is generated using the new
ESP IDF kconfig_new tool written in Python. Custom configuration for a
particular ESP32 board can be specified via the make variable SDKCONFIG.
The esp32.common.ld file is also now generated using the standard ESP IDF
ldgen.py tool.
