Add initramfs write extension

This creates a gzipped cpio archive that may be used as an initramfs.
It is hard-coded to use gzip to compress the initramfs, since it's the
most common way to do it.

This is unfortunate, since the busybox gzip utility only allows maximum
compression, which is rather slow and doesn't give progress reporting,
so you can easily think it's gotten stuck.

It's possible to use other compression formats, but they need the kernel
to be built with them supported, and in the case of lz4, unusual userland
tools to create it, since the version of lz4 supported in the kernel is
not what the standard lz4 tools produce.

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