py/py.mk: When building axtls use -Wno-all to prevent all warnings.

Building axtls gives a lot of warnings with -Wall enabled, and explicitly
disabling all of them cannot be done in a way compatible with gcc and
clang, and likely other compilers.  So just use -Wno-all to prevent all of
the extra warnings (in addition to the necessary -Wno-unused-parameter,
-Wno-uninitialized, -Wno-sign-compare and -Wno-old-style-definition).

Fixes issue #4182.