lib/vfscore: Embedded initrd

lib/vfscore: Embedded initrd

This commit introduces the ability to embed a CPIO initrd in a unikernel
image. This can be useful to hard link an initrd content to an
unikernel. An additionally provided initrd can then be used freely with the
fstab function.

Checkpatch-Ignore: AVOID_EXTERNS
Signed-off-by: Simon Kuenzer <simon@unikraft.io>
Reviewed-by: Cezar Craciunoiu <cezar.craciunoiu@unikraft.io>
Reviewed-by: Sergiu Moga <sergiu@unikraft.io>
Reviewed-by: Marco Schlumpp <marco@unikraft.io>
Approved-by: Marco Schlumpp <marco@unikraft.io>
GitHub-Closes: #1070