composefs: When using signatures, delay application until first boot

composefs: When using signatures, delay application until first boot

We can't safely apply the fs-verity with signature until we have
booted with the new initrd, because the public key that matches the
signature is loaded from it. So, instead we save the .sig file next
to the compoosefs, and on the first boot we detect that it is there, and
the composefs file isn't fs-verity, so we apply it.

Things get a bit more complex due to having to temporarily make
/sysroot read-write for the fsverity operation too.