archive: support using lz4 legacy

fixes: #27 (closed)

This MR adds support for the lz4 legacy compression format. This is the format the Linux kernel supports for LZ4, and the one Google uses with their new GKI and multiple initramfs in their new vendor_boot partition.

This MR might need more unit tests written, and I named the format lz4l for lz4 legacy, but that might not be necessary as if we only care about what formats the Linux kernel supports, there's only one, and it's legacy lz4, so I could just rename it to lz4.

I tested this by pmbootstrap build --arch aarch64 --src [path to repo], copying the generated .apk to the device chroot and installing it with apk, and running mkinitfs, and it appears to have compressed the initramfs correctly as lz4 legacy (I had already set the required variable in the deviceinfo file to select the compression type, as it was added back for the lzma support).