When building greentea tests, each test is an executable with its
own output binary path. This is also the case when a user project
produces multiple executables. But the current implementation of
post-build operations always assumes there's only one executable,
at the root of the build directory.

The post-build command depends on Mbed target, and it always takes
the the executable we build as an input file. To achieve this, we
let each Mbed target (that has a post-build command) define a function

    function(mbed_post_build_function target)

which takes a CMake executable target as an argument from which it can
get its binary path using generator expressions. It generates and adds
to the passed executable target a post-build custom command.

Notes:
* The function name needs to be exact, because CMake only supports
literal function calls - CMake can't dereference a function name from
a variable. To avoid multiple definitions of this function, each Mbed
target needs to guard it with a macro to check if the user is
building this Mbed target.
* `mbed_post_build_function()` is a function, but it is usually
defined by another macro rather than a parent function, because
nesting functions would make many variables inaccessible inside the
innermost `mbed_post_build_function()`.
* There's no more need to force regenerate images. Previously, post-
build commands were custom *targets* which always got to run, so we
force regenerated images on every build to avoid patching an image
that's already been patched once on previous build. Now post-build
commands are custom *commands* of the same executable target, and they
are only run if the executable target itself is rebuilt.