Build mbedcrypto from other directory than mbedcrypto root dir.

[kapstok]

Made some changes in CMake to make the mbedcrypto library also 'buildable' from a different directory than the mbedcrypto root directory. This is useful when a CMake project, say Project A, uses the library.

Project A has the mbedcrypto repo as a submodule in the directory ProjectA/dep/mbedcrypto.

The developer of Project A can add the library simply by adding this code to ProjectA/CMakeLists.txt:

file (GLOB MBEDCRYPTO "dep/mbedcrypto")
add_subdirectory(${MBEDCRYPTO})
target_link_libraries (ProjectA mbedcrypto)

From that point the library is linked to Project A.
The library can still be tested with mbedcrypto_tests. After calling cmake and make from the (unix) Terminal, the developer can test the library by executing:

dep/mbedcrypto.xbin64/mbedcrypto_tests

The statement is executed from the CMAKE_BINARY_DIRECTORY.

If this pull request will be approved, I'm also planning to add a wiki page to the repo that describes how to link the mbedcrypto library to your own project as developer.

[azadkuh]

@kapstok
sorry i'm so busy this week, let me check it at weekend.

[kapstok]

Not a problem. I'll see a response when you have enough time :)