linux-postmarketos-exynos4: reduce boot image size

I'm marking this as a work in progress for now as with this, the device boots to a "initramfs-extra not found" splash screen. If anyone has any ideas regarding what could be causing this I'd like to hear them.

Set module compression algorithm to xz instead of zstd and it works fine on samsung-m0 and samsung-p4note now.

Closes #1785 (closed)