Add ability to specify custom wsialloc implementations

Adds the ability to link the project with a custom wsialloc library.

This makes it possible to customize the format selection and allocation to the target system, according to its capabilities.

Change-Id: Iade5cd27aa3515d3debb7fe1f2b96453776d77be Signed-off-by: Normunds Rieksts normunds.rieksts@arm.com