main/linux-postmarketos-exynos4: compress with xz

gzip results in a boot.img that's too big (over 8 MB) for e.g. samsung-i9300, so let's replace it with something that produces smaller kernels. With this, the entire boot.img is 6,3 MB (down from 8MiB, or 8.5458944 MB) which leaves plenty of space on the 8 MB partition and prevents the recovery from being overwritten when this kernel is flashed.

Tested and works on samsung-i9300.

Closes #1023 (closed)