lib/axtls: Update to latest axtls 2.1.5 wih additional commits.

Changes are:
- update axTLS from 2.1.3 to 2.1.5
- os_port.h is now provided by the user of the library
- PLATFORM_RNG_U8 can be defined for get_random
- fix -fsanitize=undefined diagnostics

Signed-off-by: Damien George <[email protected]>

Author: dpgeorge