Improve OSAL API and remove TLV abstraction layer

[ismilak]

A possible improvement target can be removing the TLV abstraction layer from osal branch. Currently there is an abstraction layer that makes the TLV handlers portable. This is required since an image post request calls the platform specific APIs, like to store the received image block to the memory. Different platforms have different memory handlings: EFR32 stores the chunk to the bootloader slot into the flash, linux stores the chunk into the file using the filesystem API.

If the OSAL API includes the portable solution, this differences can be eliminated, and TLV abstraction layer can be removed. It would be reasonable to reduce the maintenance efforts too. For instance a handler for Image block post should be common for all the platforms.

I note that we need to find the balance and avoid a huge and unmanageable OSAL API.