Allow to override toolchain path

[mcspr]

re: #41

Hardcode tools path in Makefile.arduino to TOOLS=~/.arduino15/packages/esp8266/tools/xtensa-lx106-elf-gcc/2.5.0-4-b40a506/bin/xtensa-lx106-elf-
Replace TOOLS=../../$(TOOLS) with TOOLS=$(TOOLS) in Makefile.build-lwip2

In case of PlatformIO, toolchain is in a custom dir:

$ env PREFIX=~/.platformio/packages/toolchain-xtensa/ make

---

IDF has a slightly different idea about pathing:
https://github.com/espressif/esp-idf/blob/f5b82c5b1f33f5287f9b5f1993594bf53755a4f2/Kconfig#L38-L45
I guess it can also be that way, essentially mapping TOOLS=$(PREFIX)

[d-a-v]

worksforme Thanks !

[mcspr]

Thanks!

At a glance, there are some other relative paths that may be need to be cleaned up too, like patches and includes (both makefile and -I...).
And I also wonder, any interest in adding cmake as a build system instead of raw make?

[d-a-v]

If you think a cleaning is needed, please propose your fixes.

As of cmake I try to stand far from it (oldschool me ?).
Anyway if you have a CMakeLists.txt we can try to find room for it.

[mcspr]

So far I was just tinkering with cmake to find out the build process overall:
https://gist.github.com/mcspr/19f8f72649c358323055a80ba5f9b209
I will add another issue / pr when it actually works with multiple flavours of tcp_mss / ipv6 / features