llext: Add new methods to loader API

Introducing `llext_prepare()` and `llext_finalize()` APIs
that are invoked and the beginning and the end of the `llext_load()`
function.

The purpose of these functions is to bring possibility of initializing
loader before it is used and uninitialize or clean up when
it is no longer needed. Both functions are optional.

The buffer loader has been aligned to methods introduced in the patch.

Signed-off-by: Adam Wojasinski <awojasinski@baylibre.com>