It would not be possible to liberate a 2015 Macbook.
As of now, there are three types of computers that can run Coreboot:
Laptops made prior to 2013 (ie. upto 3rd Gen Intel CPUs)
Eg. x200 (core2duo), x201 (1st gen i5), x220 (2nd gen i3/i5/i7) and x230 (3rd gen i5/i7) thinkpad models
Laptops from system76 or purism where they specially design their motherboards to support Coreboot with more recent Intel CPUs
Intel chromebook/chromebox models which run Coreboot by default
Other than these, its not possible to run Coreboot on other motherboards.
Your Macbook Air 2015 will have two challenges. It probably runs a 5th Gen Intel CPU. I don’t think Coreboot works on motherboards with these CPUs. Additionally, the BIOS chip might be locked and not amendable to external flashing. (Generally, we would attaching a chip programming clip and flash coreboot to the SPI BIOS chip.)
The most authoritative source for motherboard support would be: