Provide instructions on how to compile library

[mluessi]

I'm trying to use mbedtls with mongoose OS compiled with different cflags for the mbedtls library.

However, the library sources ("mbedtls/library") and the "src" directory referenced in the mos.yml file are not present in this repo. I tried copying the "library" directory from ARMmbed/mbedtls version 2.16.0, but it seems like there are changes to the header files which makes the sources incompatible with the headers in this repo.

Can you provide instructions on how to compile this library from sources instead of using the pre-compiled version?