mkinitramfs: use temporary file for uncompressed cpio

A Compressor can benefit if the uncompressed size is
known at the start.

Potential benefits are less memory requirements when decoding,
and simpler framing (no "streaming mode").

This changes the pipe to create a new temporary file
containing the main cpio archive.
The final initramfs output file can then be created in
a single write (no appending).

Signed-off-by: Norbert Lange <nolange79@gmail.com>