[mbedtls] Add Mbedtls 2.27

[jeremiahpslewis]

Per request here:

• Update mbedtls to final 2.x release JuliaLang/julia#42311

[jeremiahpslewis]

@giordano I kind of wonder whether this whole mbedtls setup is worth a rewrite...

• Lots of duplicated code by each version
• Each version is kept in code, instead of just overwriting the old one (why do we do this with mbedtls and not elsewhere?)

Thoughts?

[giordano]

Lots of duplicated code by each version

Where are the duplicates? It looks to me the idea is to minimise them?

Each version is kept in code, instead of just overwriting the old one (why do we do this with mbedtls and not elsewhere?)

For some packages, most notably Julia dependencies such as mbedtls, we occasionally need to go back and rebuild an old version. We do the same for LLVM and OpenBLAS, for example

[jeremiahpslewis]

Fair point, if we want to keep around the older versions not much. Only remaining quibble would be that we have a couple of 2.24-specific statements that are being carried along in common.jl, but this probably can't be helped.